fix: check the type of the initial packet

Before this fix, the client could mark the polling transport as open
even though the handshake packet was not received properly (for
example, after a parsing error).

See also: socketio/engine.io-client@1c8cba8

Backported from 2b5dfb9

Author: darrachequesne

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

@@ -115,7 +115,7 @@ private void _onData(Object data) {
         Parser.DecodePayloadCallback callback = new Parser.DecodePayloadCallback() {
             @Override
             public boolean call(Packet packet, int index, int total) {
-                if (self.readyState == ReadyState.OPENING) {
+                if (self.readyState == ReadyState.OPENING && Packet.OPEN.equals(packet.type)) {
                     self.onOpen();
                 }