Allow adding transports before defaults (#272)

As I mentioned in #80 (comment), one could not override default transports now.

With this small change, one could set up own transport, because first transport get's selected:
https://github.com/99designs/gqlgen/blob/fb67b709af4f0c5ba6eff8222c728b076a6eb09b/graphql/handler/server.go#L98

It will allow users, for example, to introduce batching with a bramble plugin, as I also mentioned in comment: #80 (comment).

gateway.go

@@ -42,9 +42,12 @@ func (g *Gateway) UpdateSchemas(interval time.Duration) {
 func (g *Gateway) Router(cfg *Config) http.Handler {
 	mux := http.NewServeMux()
 
+	gatewayHandler := handler.New(g.ExecutableSchema)
+	for _, plugin := range g.plugins {
+		plugin.SetupGatewayHandler(gatewayHandler)
+	}
 	// Duplicated from `handler.NewDefaultServer` minus
 	// the websocket transport and persisted query extension
-	gatewayHandler := handler.New(g.ExecutableSchema)
 	gatewayHandler.AddTransport(transport.Options{})
 	gatewayHandler.AddTransport(transport.GET{})
 	gatewayHandler.AddTransport(transport.POST{})
@@ -53,10 +56,6 @@ func (g *Gateway) Router(cfg *Config) http.Handler {
 		gatewayHandler.Use(extension.Introspection{})
 	}
 
-	for _, plugin := range g.plugins {
-		plugin.SetupGatewayHandler(gatewayHandler)
-	}
-
 	mux.Handle("/query", applyMiddleware(otelhttp.NewHandler(gatewayHandler, "/query"), debugMiddleware))
 
 	for _, plugin := range g.plugins {