Inserting HTTP Header before handler

excerpt.go

func startServer(listen string) {
	r := Router{Mux: mux.NewRouter()}
	r.Bind()
	//http.Handle("/", r.Mux)
	http.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
	    w.Header().Set("Header", "Here")
		r.Mux.ServeHTTP(w, req)
	})
	log.Printf("Starting on %s", listen)
	err := http.ListenAndServe(listen, nil)
	if err != nil {
		log.Fatal("ListenAndServe: \"%s\"", err.Error())
		os.Exit(1)
	}
}