code: func (ch challenge) SetHeaders(r *http.Request, w http.ResponseWriter) {
  	header := fmt.Sprintf("Bearer realm=%q,service=%q", ch.realm, ch.service)
  
  	if ch.scope != "" {
  		header = fmt.Sprintf("%s,scope=%q", header, ch.scope)
  	}
  
  	w.Header().Set("WWW-Authenticate", header)
  }
docs: // SetHeaders sets a simple bearer challenge on the response.