title: RxSwift and UICollectionView Header
body: <p>I'm using <code>RxSwift</code> with my <code>UIViewController</code> that contains a <code>UICollectionView</code>. I have tried to add a header to my collection view but this is never called:  </p>
  
  <pre><code>func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath)
  </code></pre>