Call connection_open after websocket handshake

It seems that due to (recent?) changes in the websocket library, we
now need to call "connection_open" to flag that the websocket is now
ready to use. I've added that call just after the call to
"connection_made".

sanic/websocket.py

@@ -90,4 +90,5 @@ class WebSocketProtocol(HttpProtocol):
         )
         self.websocket.subprotocol = subprotocol
         self.websocket.connection_made(request.transport)
+        self.websocket.connection_open()
         return self.websocket