title: Unknown provider returned from registerWithProvider
body: <p>I am trying to create a custom auth provider called "cgpsauth" and I am getting "Unknown provider" when I call:</p>
  
  <pre><code>  var ru = Realm.Sync.User.registerWithProvider(
          "http://localhost:9080",
          "cgpsauth",
          accessToken,
          (error, user) =&gt; {...
  </code></pre>
  
  <p>Here is my provider which I know is being loaded because if I make a syntax error in this file I get an error when I launch the ROS:</p>
  
  <pre><code>module.exports = function(deps) {
    return class CgpsAuthProvider extends deps.BaseAuthProvider {
  
      static get name() {
        return 'custom/cgpsauth';
      }
  
      constructor(name, options, requestPromise) {
        super(name, options, requestPromise);
      }
  
      verifyIdentifier(req) {
        const token = req.body.data;
        return 30;
      }
    }
  };
  </code></pre>
  
  <p>configuration.yml</p>
  
  <pre><code> providers:
       custom/cgpsauth:
        implementation: cgpsauth.js
  </code></pre>