Use sanitised result of select_subprotocol

- Fix jupyterhub#442
- Fix jupyterhub#445

jupyter_server_proxy/handlers.py

@@ -493,7 +493,7 @@ async def start_websocket_connection():
                 request=request,
                 on_message_callback=message_cb,
                 on_ping_callback=ping_cb,
-                subprotocols=self.subprotocols,
+                subprotocols=self.select_subprotocol(self.subprotocols),
                 resolver=resolver,
             )
             self._record_activity()