title: Show mongoose aggregation result as one object
body: <p>I want to show the aggregation result as one object, like:</p>
  <pre><code>{
    ...
  }
  </code></pre>
  <p>instead of</p>
  <pre><code>[
    {
     ...
    }
  ]
  </code></pre>
  <p>like the findOne query.</p>
  <p>Is there a way to make that possible?</p>
  <p>Thanks!</p>