code: func HasHeader(header string) ResponseCondition {
  	return func(res *http.Response) error {
  		if _, ok := res.Header[header]; !ok {
  			return errors.New("response doesn't contain header: " + header)
  		}
  		return nil
  	}
  }
docs: // HasHeader returns a retry condition function.
  // The condition returns an error if the response does not have a header set.