title: Reload collectionview cells and not the section header
body: <p>When trying to create collapsible UICollectionView sections, I update the number of items in the section dependent on its state. However, doing it this way, I reload the section which also reloads the section header aswell, and I get a very weird behavior when animating my image in the section header.</p>
  
  <p>Essentially, reloading the section header when changing section items enables the UICollectionView to update the items but the section animate looks and behaves strange. 
  Without calling reloadSection, it allows for the proper animation but the items do not load.</p>
  
  <pre><code>self?.collectionView?.performBatchUpdates({ 
     let indexSet = IndexSet(integer: section)
     self?.collectionView?.reloadSections(indexSet) 
  }, completion: nil)
  </code></pre>
  
  <p>What is the fix for this?</p>