code: func postURL(source, post string, tr http.RoundTripper) ([]byte, error) {
  	client := &http.Client{
  		Transport: tr,
  	}
  	resp, err := client.Post(source, "application/octet-stream", strings.NewReader(post))
  	if err != nil {
  		return nil, fmt.Errorf("http post %s: %v", source, err)
  	}
  	defer resp.Body.Close()
  	if resp.StatusCode != http.StatusOK {
  		return nil, fmt.Errorf("http post %s: %v", source, statusCodeError(resp))
  	}
  	return ioutil.ReadAll(resp.Body)
  }
docs: // postURL issues a POST to a URL over HTTP.