code: func (d *Doer) formatUserAgent() string {
  	ua := fmt.Sprintf("%s %s (%s; %s; %s)", d.userAgent, ourUserAgent, ourUserAgentComment, runtime.GOOS, runtime.GOARCH)
  	return strings.TrimSpace(ua)
  }
docs: // formatUserAgent builds and returns the User-Agent string to use in requests.