add tests for Parser

src/main/java/com/github/nkzawa/engineio/client/Socket.java

@@ -249,7 +249,7 @@ public abstract class Socket extends Emitter {
                                     transport[0].off(Transport.EVENT_ERROR, onerror);
                                     self.emit(EVENT_UPGRADE, transport);
                                     self.setTransport(transport[0]);
-                                    Packet packet = new Packet(Packet.UPGRADE, null);
+                                    Packet packet = new Packet(Packet.UPGRADE);
                                     transport[0].send(new Packet[]{packet});
                                     transport[0] = null;
                                     self.upgrading = false;

src/main/java/com/github/nkzawa/engineio/parser/Packet.java

@@ -15,14 +15,12 @@ public class Packet {
     public String type;
     public String data;
 
+    public Packet(String type) {
+        this(type, null);
+    }
+
     public Packet(String type, String data) {
         this.type = type;
         this.data = data;
     }
-
-    @Override
-    public String toString() {
-        return String.format("{\"type\": \"%s\", \"data\": \"%s\"}", this.type, this.data);
-    }
-
 }

src/main/java/com/github/nkzawa/engineio/parser/Parser.java

@@ -114,6 +114,7 @@ public class Parser {
     }
 
     public static interface DecodePayloadCallback {
+
         public boolean call(Packet packet, int index, int total);
     }
 }