feat: add an extraHeaders option

Similar to the option of the JS client:

```java
opts = new Socket.Options();
opts.extraHeaders = singletonMap("authorization", singletonList("bearer abcd"));
socket = new Socket(opts);
```

Note: the refactor of the options (similar to [1]) will be done in a
future step.

[1] 5f47a50ee5

src/main/java/io/socket/engineio/client/Socket.java

@@ -130,6 +130,7 @@ public class Socket extends Emitter {
     private Future pingTimeoutTimer;
     private okhttp3.WebSocket.Factory webSocketFactory;
     private okhttp3.Call.Factory callFactory;
+    private final Map<String, List<String>> extraHeaders;
 
     private ReadyState readyState;
     private ScheduledExecutorService heartbeatScheduler;
@@ -221,6 +222,7 @@ public class Socket extends Emitter {
             }
             webSocketFactory = defaultOkHttpClient;
         }
+        this.extraHeaders = opts.extraHeaders;
     }
 
     public static void setDefaultOkHttpWebSocketFactory(okhttp3.WebSocket.Factory factory) {
@@ -293,6 +295,7 @@ public class Socket extends Emitter {
         opts.policyPort = options != null ? options.policyPort : this.policyPort;
         opts.callFactory = options != null ? options.callFactory : this.callFactory;
         opts.webSocketFactory = options != null ? options.webSocketFactory : this.webSocketFactory;
+        opts.extraHeaders = this.extraHeaders;
 
         Transport transport;
         if (WebSocket.NAME.equals(name)) {

src/main/java/io/socket/engineio/client/Transport.java

@@ -1,6 +1,7 @@
 package io.socket.engineio.client;
 
 
+import java.util.List;
 import java.util.Map;
 
 import io.socket.emitter.Emitter;
@@ -43,6 +44,7 @@ public abstract class Transport extends Emitter {
     protected ReadyState readyState;
     protected WebSocket.Factory webSocketFactory;
     protected Call.Factory callFactory;
+    protected Map<String, List<String>> extraHeaders;
 
     public Transport(Options opts) {
         this.path = opts.path;
@@ -55,6 +57,7 @@ public abstract class Transport extends Emitter {
         this.socket = opts.socket;
         this.webSocketFactory = opts.webSocketFactory;
         this.callFactory = opts.callFactory;
+        this.extraHeaders = opts.extraHeaders;
     }
 
     protected Transport onError(String msg, Exception desc) {
@@ -146,5 +149,6 @@ public abstract class Transport extends Emitter {
         protected Socket socket;
         public WebSocket.Factory webSocketFactory;
         public Call.Factory callFactory;
+        public Map<String, List<String>> extraHeaders;
     }
 }

src/main/java/io/socket/engineio/client/transports/PollingXHR.java

@@ -43,6 +43,7 @@ public class PollingXHR extends Polling {
         }
         opts.uri = this.uri();
         opts.callFactory = this.callFactory;
+        opts.extraHeaders = this.extraHeaders;
 
         Request req = new Request(opts);
 
@@ -72,6 +73,7 @@ public class PollingXHR extends Polling {
         Request.Options opts = new Request.Options();
         opts.method = "POST";
         opts.data = data;
+        opts.extraHeaders = this.extraHeaders;
         Request req = this.request(opts);
         final PollingXHR self = this;
         req.on(Request.EVENT_SUCCESS, new Emitter.Listener() {
@@ -150,6 +152,7 @@ public class PollingXHR extends Polling {
         private String data;
 
         private Call.Factory callFactory;
+        private Map<String, List<String>> extraHeaders;
         private Response response;
         private Call requestCall;
 
@@ -158,13 +161,16 @@ public class PollingXHR extends Polling {
             this.uri = opts.uri;
             this.data = opts.data;
             this.callFactory = opts.callFactory != null ? opts.callFactory : new OkHttpClient();
+            this.extraHeaders = opts.extraHeaders;
         }
 
         public void create() {
             final Request self = this;
             if (LOGGABLE_FINE) logger.fine(String.format("xhr open %s: %s", this.method, this.uri));
             Map<String, List<String>> headers = new TreeMap<String, List<String>>(String.CASE_INSENSITIVE_ORDER);
-
+            if (this.extraHeaders != null) {
+                headers.putAll(this.extraHeaders);
+            }
             if ("POST".equals(this.method)) {
                 headers.put("Content-type", new LinkedList<String>(Collections.singletonList(TEXT_CONTENT_TYPE)));
             }
@@ -255,6 +261,7 @@ public class PollingXHR extends Polling {
             public String method;
             public String data;
             public Call.Factory callFactory;
+            public Map<String, List<String>> extraHeaders;
         }
     }
 }

src/main/java/io/socket/engineio/client/transports/WebSocket.java

@@ -35,6 +35,9 @@ public class WebSocket extends Transport {
 
     protected void doOpen() {
         Map<String, List<String>> headers = new TreeMap<String, List<String>>(String.CASE_INSENSITIVE_ORDER);
+        if (this.extraHeaders != null) {
+            headers.putAll(this.extraHeaders);
+        }
         this.emit(EVENT_REQUEST_HEADERS, headers);
 
         final WebSocket self = this;

src/test/java/io/socket/engineio/client/ServerConnectionTest.java

@@ -10,11 +10,14 @@ import org.junit.runners.JUnit4;
 
 import java.net.URISyntaxException;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.LinkedBlockingQueue;
 
+import static java.util.Collections.singletonList;
+import static java.util.Collections.singletonMap;
 import static org.hamcrest.CoreMatchers.is;
 import static org.hamcrest.CoreMatchers.not;
 import static org.hamcrest.CoreMatchers.notNullValue;
@@ -168,6 +171,38 @@ public class ServerConnectionTest extends Connection {
         socket.close();
     }
 
+    @Test(timeout = TIMEOUT)
+    public void pollingHeaders_withExtraHeadersOption() throws URISyntaxException, InterruptedException {
+        final BlockingQueue<String> messages = new LinkedBlockingQueue<String>();
+
+        Socket.Options opts = createOptions();
+        opts.transports = new String[] {Polling.NAME};
+        opts.extraHeaders = singletonMap("X-EngineIO", singletonList("bar"));
+
+        socket = new Socket(opts);
+        socket.on(Socket.EVENT_TRANSPORT, new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                Transport transport = (Transport)args[0];
+                transport.on(Transport.EVENT_RESPONSE_HEADERS, new Emitter.Listener() {
+                    @Override
+                    public void call(Object... args) {
+                        @SuppressWarnings("unchecked")
+                        Map<String, List<String>> headers = (Map<String, List<String>>)args[0];
+                        List<String> values = headers.get("X-EngineIO");
+                        messages.offer(values.get(0));
+                        messages.offer(values.get(1));
+                    }
+                });
+            }
+        });
+        socket.open();
+
+        assertThat(messages.take(), is("hi"));
+        assertThat(messages.take(), is("bar"));
+        socket.close();
+    }
+
     @Test(timeout = TIMEOUT)
     public void websocketHandshakeHeaders() throws URISyntaxException, InterruptedException {
         final BlockingQueue<String> messages = new LinkedBlockingQueue<String>();
@@ -206,6 +241,38 @@ public class ServerConnectionTest extends Connection {
         socket.close();
     }
 
+    @Test(timeout = TIMEOUT)
+    public void websocketHandshakeHeaders_withExtraHeadersOption() throws URISyntaxException, InterruptedException {
+        final BlockingQueue<String> messages = new LinkedBlockingQueue<String>();
+
+        Socket.Options opts = createOptions();
+        opts.transports = new String[] {WebSocket.NAME};
+        opts.extraHeaders = singletonMap("X-EngineIO", singletonList("bar"));
+
+        socket = new Socket(opts);
+        socket.on(Socket.EVENT_TRANSPORT, new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                Transport transport = (Transport)args[0];
+                transport.on(Transport.EVENT_RESPONSE_HEADERS, new Emitter.Listener() {
+                    @Override
+                    public void call(Object... args) {
+                        @SuppressWarnings("unchecked")
+                        Map<String, List<String>> headers = (Map<String, List<String>>)args[0];
+                        List<String> values = headers.get("X-EngineIO");
+                        messages.offer(values.get(0));
+                        messages.offer(values.get(1));
+                    }
+                });
+            }
+        });
+        socket.open();
+
+        assertThat(messages.take(), is("hi"));
+        assertThat(messages.take(), is("bar"));
+        socket.close();
+    }
+
     @Test(timeout = TIMEOUT)
     public void rememberWebsocket() throws InterruptedException {
         final BlockingQueue<Object> values = new LinkedBlockingQueue<Object>();