Crash on reloadData in collection view when header is offscreen

Hello again @jamztang - found some issues with reloading data on the collection view.

The CSStickyHeaderFlowLayout was calling its base class UICollectionViewLayout layoutAttributesForSupplementaryViewOfKind: method to get layout attributes. That base class knows nothing about the CSStickyHeaderParallaxHeader kind of supplementary view, and was returning nil.

Fix:

- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewLayoutAttributes *attributes = [super layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
    if (!attributes && [kind isEqualToString:CSStickyHeaderParallaxHeader]) {
        attributes = [CSStickyHeaderFlowLayoutAttributes layoutAttributesForSupplementaryViewOfKind:kind withIndexPath:indexPath];
    }
    return attributes;
}