Use sanitised result of selected_subprotocol

- Fix #442
- Fix #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.selected_subprotocol,
                 resolver=resolver,
             )
             self._record_activity()
@@ -530,14 +530,6 @@ def check_xsrf_cookie(self):
         Defer to proxied apps.
         """
 
-    def select_subprotocol(self, subprotocols):
-        """Select a single Sec-WebSocket-Protocol during handshake."""
-        self.subprotocols = subprotocols
-        if isinstance(subprotocols, list) and subprotocols:
-            self.log.debug(f"Client sent subprotocols: {subprotocols}")
-            return subprotocols[0]
-        return super().select_subprotocol(subprotocols)
-
 
 class LocalProxyHandler(ProxyHandler):
     """