esponse.request().newBuilder()
        .header("Authorization", credential)
        .build();
 </code></pre>

 <p>When reactive authentication is requested by a proxy server, the response code is 407 and the
 implementation should respond with a new request that sets the "Proxy-Authorization" header.
 <pre>   <code>

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

    String credential = Credentials.basic(...)
  