cate. If so it is likely that further attempts will not be useful and the authenticator
 should give up.

 <p>When reactive authentication is requested by an origin web server, the response code is 401
 and the implementation should respond with a new request that sets the "Authorization" header.
 <pre>   <code>

    if (response.request().header("Authorization") != null) {
      return null; // Give up, we've already failed to authenticate.
    }

    String credential = Credentials.basic(...)
    return r