code: func (r *ResponseRecorder) Flush() {
  	if f, ok := r.ResponseWriter.(http.Flusher); ok {
  		f.Flush()
  	} else {
  		panic(NonFlusherError{Underlying: r.ResponseWriter}) // should be recovered at the beginning of middleware stack
  	}
  }
docs: // Flush implements http.Flusher. It simply wraps the underlying
  // ResponseWriter's Flush method if there is one, or does nothing.