userAgent := defaultUserAgent
	if r.Header.has("User-Agent") {
		userAgent = r.Header.Get("User-Agent")
	}
	if userAgent != "" {
		userAgent = headerNewlineToSpace.Replace(userAgent)
		userAgent = textproto.TrimString(userAgent)
		_, err = fmt.Fprintf(w, "User-Agent: %s\r\n", userAgent)
		if err != nil {
			return err
		}
		if trace != nil && trace.WroteHeaderField != nil {