Fix bug where subscribe event is sent twice.

Caused by reconnect behavior wherein it reconnects to the previous
channels, and the fact that the ConnectAsync() method did not wait for
the connection established event to be raised.

Solved by simply having ConnectAsync() wait for the connection
established event.

Pusher/Pusher.cs

@@ -77,6 +77,14 @@ public async Task ConnectAsync()
 					                      ApplicationKey)));
 			_connection.OnData += ReceivedEvent;
 			await _connection.Open();
+
+			// wait for the connection established event before returning
+			var completionSource = new TaskCompletionSource<string>();
+			GenericEventEmittedHandler<ConnectionEstablishedEventArgs> eventHandler =
+				(sender, e) => completionSource.SetResult(e.DataObject.SocketId);
+			GetEventSubscription<ConnectionEstablishedEventArgs>().EventEmitted += eventHandler;
+			await completionSource.Task;
+			GetEventSubscription<ConnectionEstablishedEventArgs>().EventEmitted -= eventHandler;
 		}
 
 		public void AddContract(IEventContract contract)