code: func ResponseChallenges(resp *http.Response) []Challenge {
  	if resp.StatusCode == http.StatusUnauthorized {
  		// Parse the WWW-Authenticate Header and store the challenges
  		// on this endpoint object.
  		return parseAuthHeader(resp.Header)
  	}
  
  	return nil
  }
docs: // ResponseChallenges returns a list of authorization challenges
  // for the given http Response. Challenges are only checked if
  // the response status code was a 401.