title: UICollectionViewFlowLayout not invalidating right on orientation change
body: <p>i have a UICollectionView with a UICollectionViewFlowLayout. I also implement the UICollectionViewDelegateFlowLayout protocol.</p>
  
  <p>In my datasource i have a bunch of UIViewControllers which respond to a custom protocol so i can ask for their size and some other stuff.</p>
  
  <p>In in the FlowLayout delegate when it asks for the sizeForItemAtIndexPath:, i return the item size which i get from my protocol. The ViewControllers which implement my protocol return a different item size depending on the orientation. </p>
  
  <p>Now if i change the device orientation from portrait to landscape theres no problem (Items are larger in landscape) but if i change it back, i get this warning:</p>
  
  <pre><code>    the item width must be less that the width of the UICollectionView minus the section insets left and right values.
      Please check the values return by the delegate.
  </code></pre>
  
  <p>It still works but i don't like it to get warnings so maybe you can tell me what i am doing wrong. Also there is another problem. If i don't tell my collectionViews collectionViewLayout to invalidate in willAnimateRotationToInterfaceOrientation: the sizeForItemAtIndexPath: is never called.</p>
  
  <p>Hope you understand what i mean. If you need additional information let me know :)</p>