title: UICollectionViewFlowLayout adding extra space before first section header
body: <p>Can anyone explain why UICollectionViewFlowLayout is adding extra space before the first section header in the following? The yellow border is the <code>UICollectionView</code> frame. The blue borders are the <code>UICollectionReusableView</code> section header frames:</p>
  
  <p><img src="https://i.stack.imgur.com/doUhS.png" alt="enter image description here"></p>
  
  <p>The extra space is only appearing above the <em>first</em> section header, which is odd. </p>
  
  <p>As far as I can tell, I'm zeroing everything out. This is how I instantiate the flow layout and collection view: </p>
  
  <pre><code>UICollectionViewFlowLayout *flowLayout = [[UICollectionViewFlowLayout alloc] init];
  flowLayout.itemSize = [BCFriendCell defaultSize];
  flowLayout.minimumInteritemSpacing = 0.0f;
  flowLayout.minimumLineSpacing = 0.0f;
  flowLayout.headerReferenceSize = CGSizeMake(kScreenWidth, 63.0f);
  flowLayout.footerReferenceSize = CGSizeZero;
  flowLayout.sectionInset = UIEdgeInsetsZero;
  
  CGRect collectionViewFrame = ...
  collectionView = [[UICollectionView alloc] initWithFrame:collectionViewFrame collectionViewLayout:flowLayout];
  
  collectionView.contentInset = UIEdgeInsetsZero;
  [self.view addSubview:collectionView];
  
  NSLog(@"contentOffset: %@", NSStringFromCGPoint(collectionView.contentOffset));
  </code></pre>
  
  <p>That last logging line prints out the expected </p>
  
  <pre><code>2014-09-30 19:23:28.974 MyApp[4468:60b] contentOffset: {0, 0}
  </code></pre>
  
  <p>Thanks for the help. </p>