on Required</code> response that has a <code>Proxy-Authenticate:
 OkHttp-Preemptive</code> challenge. The proxy authenticator may return either either an authenticated
 request, or null to connect without authentication.
 <pre>   <code>
    for (Challenge challenge : response.challenges()) {
      // If this is preemptive auth, use a preemptive credential.
      if (challenge.scheme().equalsIgnoreCase("OkHttp-Preemptive")) {
        return response.request().newBuilder()
            .header("Proxy-Authoriza