code: func CtxDomain(r *http.Request) string {
  	val := r.Context().Value(domainCtx)
  	if val == nil {
  		return ""
  	}
  	return val.(string)
  }
docs: // CtxDomain returns a domain that's been set on the request. Use it to get the
  // domain set on a 401 error handler.