title: CollectionView header doesn't resize on auto rotate
body: <p>I have a headerView in my CollectionView and have resized the headerView programmatically based on the text size of the Label. On rotating to landscape orientation the reusable header view doesn't resize automatically but on scrolling it resizes itself to give the intended result. 
  <img src="https://i.stack.imgur.com/ufvsN.png" alt="In portrait mode">
  <img src="https://i.stack.imgur.com/3i3Zu.png" alt="In landscape mode before scrolling"></p>
  
  <p><img src="https://i.stack.imgur.com/Qwpv4.png" alt="In Landscape mode after scrolling"></p>
  
  <p>Below is the snippet I have used to resize my header.</p>
  
  <pre><code>override func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -&gt; UICollectionReusableView {
  
      switch kind{
  
      case UICollectionElementKindSectionHeader:
  
          let headerView = collectionView.dequeueReusableSupplementaryViewOfKind(kind, withReuseIdentifier: "HeaderView", forIndexPath: indexPath) as! CollectionViewHeader
          var headerString = westBeaches[indexPath.section] as NSString
          var newSize: CGSize = headerString.sizeWithAttributes([NSFontAttributeName:headerView.headerText.font])
          headerView.frame.size.width = newSize.width + 20
          headerView.layer.cornerRadius = 15
          headerView.headerText.text = westBeaches[indexPath.section]
          headerView.center.x = collectionView.center.x
          headerView.alpha = 0.7
          return headerView
  
      default:
  
          assert(false, "Unexpected element Kind")
  
      }
  
  }
  </code></pre>
  
  <p>CollectionViewHeader is a custom class inheriting UIcollectionReusableView and contains headerText as UILabel. Is there any way to prevent the reusable view from going back to its original size when the orientation changes?</p>