Bail out early if HTTP is spoken to /ws endpoint

server/server.go

@@ -717,6 +717,9 @@ func (s *Server) handleSubscribeHTTP(w http.ResponseWriter, r *http.Request, v *
 }
 
 func (s *Server) handleSubscribeWS(w http.ResponseWriter, r *http.Request, v *visitor) error {
+	if r.Header.Get("Upgrade") != "websocket" {
+		return errHTTPBadRequestWebSocketsUpgradeHeaderMissing
+	}
 	if err := v.SubscriptionAllowed(); err != nil {
 		return errHTTPTooManyRequestsLimitSubscriptions
 	}