node: relax websocket connection header check (#21646)

This makes it accept the "upgrade,keep-alive" header value, which
apparently is a thing.

node/rpcstack.go

@@ -313,7 +313,7 @@ func (h *httpServer) wsAllowed() bool {
 // isWebsocket checks the header of an http request for a websocket upgrade request.
 func isWebsocket(r *http.Request) bool {
 	return strings.ToLower(r.Header.Get("Upgrade")) == "websocket" &&
-		strings.ToLower(r.Header.Get("Connection")) == "upgrade"
+		strings.Contains(strings.ToLower(r.Header.Get("Connection")), "upgrade")
 }
 
 // NewHTTPHandlerStack returns wrapped http-related handlers