title: cellForItemAtIndexPath returns nil after force scrolling to make it visible
body: <p>So I am trying to write some code that scrolls a collection view to a certain index, then pulls a reference to the cell and does some logic.  However I've noticed if that cell wasn't presently visible prior to the scroll, the <code>cellForItemAtIndexPath</code> call will return nil, causing the rest of my logic to fail.</p>
  
  <pre><code>[_myView scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:index 
                                                       inSection:0] 
                  atScrollPosition:UICollectionViewScrollPositionTop
                          animated:NO];
  
  //Tried with and without this line, thinking maybe this would trigger a redraw
  [_myView reloadData];
  
  //returns nil if cell was off-screen before scroll
  UICollectionViewCell *cell = 
    [_myView cellForItemAtIndexPath:
      [NSIndexPath indexPathForItem:index inSection:0]];
  </code></pre>
  
  <p>Is there some other method I have to call to cause the <code>cellForItemAtIndexPath</code> to return something for a cell that suddenly came into view as a result of the scroll immediately preceding it?</p>