title: Laravel + Jenssegers\Mongodb: 'WhereHas' and 'Has' returns empty collection
body: <p>I'm mainly working on two models right now, <code>Form</code> and <code>Notification</code>, and a many-to-many relationship is set up and working for most Eloquent commands, except for <code>whereHas</code> and <code>has</code>. Both just return an empty array, <code>[]</code>.</p>
  
  <p>It seems like the developer has had trouble with <a href="https://github.com/jenssegers/laravel-mongodb/issues/131" rel="noreferrer">getting this to work in the past</a>, but seems to have solved it <a href="https://github.com/jenssegers/laravel-mongodb/issues/182" rel="noreferrer">here</a>.</p>
  
  <p>Here's a sample of what I have so far, and what I've tried:</p>
  
  <blockquote>
    <p>Form.php</p>
  </blockquote>
  
  <pre><code>class Form extends Eloquent {
      protected $connection = 'mongodb';
  
      public function notifications(){
          return $this-&gt;belongsToMany('App\Api\Forms\Notification', null, 'form_ids', 'notification_ids');
      }
  
  }
  </code></pre>
  
  <hr>
  
  <blockquote>
    <p>Notification.php</p>
  </blockquote>
  
  <pre><code>class Notification extends Eloquent {
      protected $connection = 'mongodb';
  
      public function forms()
      {
          return $this-&gt;belongsToMany('App\Api\Forms\Form', null, 'notification_ids', 'form_ids');
      }    
  }
  </code></pre>
  
  <hr>
  
  <blockquote>
    <p>NotificationController.php</p>
  </blockquote>
  
  <pre><code>&lt;?php
  
  namespace App\Http\Controllers;
  use App\Api\Forms\Notification;
  use App\Api\Forms\Form;
  
  class NotificationController extends Controller
  {
  
      public function getByFormTitle($form_title)
      {
          // This code retuns the relationship as expected.
          // Any forms that are assigned to it are returned.
          // $n = Notification::first();
          // $n-&gt;forms()-&gt;get();
  
          // This also returns the relationship correctly, same as before.
          // $f = Form::first();
          // $f-&gt;notifications()-&gt;get();
  
  
          // Nearly identical to the Laravel docs. This returns an empty array, []
          $notifications = Notification::whereHas('forms', function ($query) use ($form_title) {
              $query-&gt;where('form_title', $form_title);
          })-&gt;get();
  
          return $notifications;
      }
  
  }
  </code></pre>
  
  <p>I get the same result if I use <code>Notification::has('form')-&gt;get()</code>.</p>
  
  <p>So my question is:</p>
  
  <p>Is it possible to use <code>whereHas</code> and <code>has</code> in <code>Jenssegers\Mongodb Eloquent</code>? Do I have to use different syntax than the official Laravel documentation for it, or do I have to make a raw Mongo query for this?</p>