fix nullpoint

https://github.com/mrniko/netty-socketio/issues/596

reason:
WebSocketTransport line 144 : Packet packet = new Packet(PacketType.MESSAGE); //subType =null
PacketEncoder line 288: byte subType = toChar(packet.getSubType().getValue()); //NullPointerException

fix
WebSocketTransport
Packet packet = new Packet(PacketType.MESSAGE);
packet.setSubType(PacketType.MESSAGE);

src/main/java/com/corundumstudio/socketio/transport/WebSocketTransport.java

@@ -142,6 +142,7 @@ public class WebSocketTransport extends ChannelInboundHandlerAdapter {
         final  Channel channel = ctx.channel();
         ClientHead client = clientsBox.get(channel);
         Packet packet = new Packet(PacketType.MESSAGE);
+        packet.setSubType(PacketType.MESSAGE);
         if (client != null && client.isTransportChannel(ctx.channel(), Transport.WEBSOCKET)) {
             log.debug("channel inactive {}", client.getSessionId());
             client.onChannelDisconnect();