title: How to observe credential in AuthenticationInterceptor from Alamofire
body: <p>I'm using <code>AuthenticationInterceptor</code> with a custom <code>Authenticator</code> to handle the token refresh.</p>
  <p>Since we are migrating from an old network stack to Alamofire, I need to keep track of the <code>credential</code> property in <code>AuthenticationInterceptor</code>  to feed that into the old network stack.</p>
  <p>Is there anyway to observe the change of <code>credential</code>.</p>
  <p>Unfortunately, <code>AuthenticationInterceptor</code> is not <code>open</code> for subclassing.</p>