Only apply cell attrs to cells

If a footer attr is applied to a cell, sometimes we get a duplicate cell, sometimes we crash, neither is desirable.

This is a better fix than https://github.com/educreations/PSTCollectionView/commit/838ec5e3133e90bce53e0872c678536b29247c74 which stopped working with https://github.com/scutdavy/PSTCollectionView/commit/a73d901ae707571bd6f3a3b2f884ef27fafc6e11