title: UICollectionView setContentOffset assertion failure
body: <p>I'm using Xcode 6 and iOS 8.
  I want to set content offset of my Collection View every time I load View Controller.</p>
  
  <pre><code>CGFloat offsetY = [self.collectionView layoutAttributesForSupplementaryElementOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath].frame.origin.y;    
  [self.collectionView setContentOffset:CGPointMake(0, MIN(self.collectionView.contentSize.height-self.collectionView.bounds.size.height, offsetY-headerHeight)) animated:NO];
  </code></pre>
  
  <p>But I receive this error: <code>*** Assertion failure in -[UICollectionViewData validateLayoutInRect:]</code></p>
  
  <p>What I've tried: <code>[self.collectionView prepare layout];</code>
  Putting this code in <code>-viewDidLayoutSubviews</code> changed nothing.</p>