code: func NewResponseWriter(rw http.ResponseWriter) ResponseWriter {
  	newRw := responseWriter{rw, 0, 0, nil}
  	if cn, ok := rw.(http.CloseNotifier); ok {
  		return &closeNotifyResponseWriter{newRw, cn}
  	}
  	return &newRw
  }
docs: // NewResponseWriter creates a ResponseWriter that wraps an http.ResponseWriter