code: func (d *Doer) Head(url string) (*http.Response, error) {
  	req, err := d.newRequest(http.MethodHead, url, nil)
  	if err != nil {
  		return nil, err
  	}
  
  	return d.do(req, nil)
  }
docs: // Head performs a HEAD request with a proper User-Agent string.
  // The response body (resp.Body) is already closed when this function returns.