title: Prevent UICollectionViewFlowLayout from calculating its own sectionInsets with variable size items
body: <p>When I specify sectionInsets, for example:</p>
  
  <pre><code>flowLayout.sectionInset = UIEdgeInsetsMake(80, 50, 10, 20);
  </code></pre>
  
  <p>And all items in my UICollectionView are the same size, the sectionInsets are applied as expected.</p>
  
  <p>When all items in the UICollectionView are <em>not</em> the same size, the sectionInsets are not applied - the flow layout seems to calculate and use its own insets. Is there any way to prevent this behaviour? We'd like the given sectionInset values to hold even with variable item sizes.</p>