Force legacy ws subprotocol when using gateway

Currently, ws subprotocols should be the same for notebook client and
gateway since there is no serializer/deserializer between the 2 streams
if subprotocols differ.

jupyter_server/gateway/connections.py

@@ -12,7 +12,7 @@
 from tornado.escape import json_decode, url_escape, utf8
 from tornado.httpclient import HTTPRequest
 from tornado.ioloop import IOLoop
-from traitlets import Bool, Instance, Int
+from traitlets import Bool, Instance, Int, Unicode
 
 from ..services.kernels.connection.base import BaseKernelWebsocketConnection
 from ..utils import url_path_join
@@ -30,6 +30,10 @@ class GatewayWebSocketConnection(BaseKernelWebsocketConnection):
 
     retry = Int(0)
 
+    # When opening ws connection to gateway, server already negotiated subprotocol with notebook client.
+    # Same protocol must be used for client and gateway, so legacy ws subprotocol for client is enforced here.
+    kernel_ws_protocol = Unicode("")
+
     async def connect(self):
         """Connect to the socket."""
         # websocket is initialized before connection