Go语言：http: superfluous response.WriteHeader call from

 在同一次请求响应过程中，只能调用一次WriteHeader(code int)，否则会有一条日志输出“http: superfluous response.WriteHeader call from”。如下所示：

func handler(w http.ResponseWriter, r *http.Request) {

	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	w.WriteHeader(200)
	w.WriteHeader(200) //重复调用

	fmt.Fprintf(w, "Hello")
}