code: func (s *SpdyRoundTripper) proxyAuth(proxyURL *url.URL) string {
  	if proxyURL == nil || proxyURL.User == nil {
  		return ""
  	}
  	credentials := proxyURL.User.String()
  	encodedAuth := base64.StdEncoding.EncodeToString([]byte(credentials))
  	return fmt.Sprintf("Basic %s", encodedAuth)
  }
docs: // proxyAuth returns, for a given proxy URL, the value to be used for the Proxy-Authorization header