title: UICollectionReusableView header possible bug
body: <p>I have a custom <code>UICollectionReusableView</code> with a single label inside: the label has leading, trailing, top and bottom constraints set to 8.</p>
  
  <p>On the controller, I register the cell and then:</p>
  
  <pre><code>func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -&gt; CGSize {
      return self.sectionSizeForIndex(section)
  }
  
  func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -&gt; UICollectionReusableView {
      let cell = self.collectionView.dequeueReusableSupplementaryViewOfKind(UICollectionElementKindSectionHeader, withReuseIdentifier: "cellID", forIndexPath: indexPath) as! FooCollectionViewCell
      cell.setupLabelWithText("Lorem ipsum")
      return cell
  }
  </code></pre>
  
  <p>For debugging reasons, I show both the borders of the cell and the borders of the inner label, with different colors.</p>
  
  <p>Here's what happens: The cell got the right frame, but the inner label seems not update constraints according to its parent view (cell).</p>
  
  <p>On "Debug View Hierarchy", I see that also <code>cell.contentView</code> doesn't update itself -> I suppose that is this the reason why the label doesn't update. </p>
  
  <p>In cell awake from nib:</p>
  
  <pre><code>override func awakeFromNib() {
      super.awakeFromNib()
      self.label.numberOfLines = 0
      self.autoresizingMask = [.FlexibleWidth, .FlexibleHeight]
      self.translatesAutoresizingMaskIntoConstraints = true
  }
  </code></pre>