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
This fixes NullPointerException when response doesn't have content type. Exception stack trace:
```
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String okhttp3.MediaType.toString()' on a null object reference
at io.socket.engineio.client.transports.PollingXHR$Request.onLoad(PollingXHR.java:271)
at io.socket.engineio.client.transports.PollingXHR$Request.access$700(PollingXHR.java:148)
at io.socket.engineio.client.transports.PollingXHR$Request$1.onResponse(PollingXHR.java:232)
at okhttp3.RealCall$AsyncCall.execute(RealCall.java:141)
at okhttp3.internal.NamedRunnable.run(NamedRunnable.java:32)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1162)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:636)
at java.lang.Thread.run(Thread.java:764)
```
Fatal Exception: java.lang.IllegalStateException: closed
at okhttp3.internal.ws.RealWebSocket.sendMessage(RealWebSocket.java:107)
at io.socket.engineio.client.transports.WebSocket$4.call(WebSocket.java:189)
at io.socket.engineio.parser.Parser.encodePacket(Parser.java:63)
at io.socket.engineio.parser.Parser.encodePacket(Parser.java:42)
at io.socket.engineio.client.transports.WebSocket.write(WebSocket.java:184)
at io.socket.engineio.client.Transport$3.run(Transport.java:108)
at io.socket.thread.EventThread.exec(EventThread.java:55)
at io.socket.engineio.client.Transport.send(Transport.java:103)
at io.socket.engineio.client.Socket.flush(Socket.java:615)
at io.socket.engineio.client.Socket.onDrain(Socket.java:606)
at io.socket.engineio.client.Socket.access$1100(Socket.java:31)
at io.socket.engineio.client.Socket$6.call(Socket.java:308)
at io.socket.emitter.Emitter.emit(Emitter.java:117)
at io.socket.engineio.client.transports.WebSocket$3$1.run(WebSocket.java:171)
at io.socket.thread.EventThread$2.run(EventThread.java:80)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)
