Fix invalid cast.

Author: Michael Mortensen

providers/netty4/src/main/java/org/asynchttpclient/netty/handler/WebSocketProtocol.java

@@ -206,7 +206,7 @@ public void onClose(NettyResponseFuture<?> future) {
         logger.trace("onClose {}");
 
         try {
-            WebSocketUpgradeHandler h = WebSocketUpgradeHandler.class.cast(future);
+            WebSocketUpgradeHandler h = WebSocketUpgradeHandler.class.cast(future.getAsyncHandler());
             NettyWebSocket webSocket = NettyWebSocket.class.cast(h.onCompleted());
 
             logger.trace("Connection was closed abnormally (that is, with no close frame being sent).");