Ensure a supplementary view is of kind _ASCollectionReusableView before setting its layoutAttributes

[nguyenhuy]

Fixes #29.

[nguyenhuy]

@ay8s Could you please help test this diff? Thank you!

[ay8s]

Working for me @nguyenhuy! 👍

[Adlai-Holler]

Hmm if we're not sure that the supplementary view is an _ASCollectionReusableView – we should investigate further why that's happening because it's not expected – we should change this method signature to willDisplaySupplementaryView:(UICollectionReusableView *) and then class-check, and then cast before setting the layout attributes.

[nguyenhuy]

@Adlai-Holler It's not an _ASCollectionResuableView under IGListKit mode, as well as interop (here).

[Adlai-Holler]

Under IGListKit it should be an _ASCollectionReusableView and good catch, I had forgotten we support interop for supplementaries.

[plarson]

Shouldn't this be fixed inside -[ASIGListSupplementaryViewSourceMethods viewForSupplementaryElementOfKind:atIndex:sectionController]?

[plarson]

I made this pull request, it's a fix for the original issue: #40

[nguyenhuy]

Closing in favor of #40. Thanks, @plarson!