Fix XCode warning about cached attributes

[crrobinson14]

XCode 7 / iOS 9 (perhaps earlier?) reports UICollectionViewFlowLayout has cached frame mismatch about the above pattern. Working from a copy resolves the issue.

[MattCBowman]

Thanks @crrobinson14! I'll try to get this integrated in the next few days.

[tcheard]

Using the copy function on [super layoutAttributesForElementsInRect:rect] only does a shallow copy and still triggers the warning.

See: http://stackoverflow.com/a/33172179/2709382

NSArray* attributesToReturn = [[NSArray alloc] initWithArray:[super layoutAttributesForElementsInRect:rect] copyItems:YES]; is better.

The copy function seems sufficient for [super layoutAttributesForItemAtIndexPath:indexPath]