code: func RoundtripWithConn(conn net.Conn) (string, error) {
  	_, err := fmt.Fprintf(conn, "GET / HTTP/1.1\r\nHost: 127.0.0.1\r\n\r\n")
  	if err != nil {
  		return "", err
  	}
  
  	re, err := http.ReadResponse(bufio.NewReader(conn), nil)
  	if err != nil {
  		return "", err
  	}
  	defer re.Body.Close()
  	out, err := ioutil.ReadAll(re.Body)
  	if err != nil {
  		return "", err
  	}
  	return string(out), nil
  }
docs: // RoundtripWithConn uses HTTP GET on the existing connection,
  // used in tests as it only performs GET request on /