Currently within `ASCollectionView`, there is an assumption that there will always be a static number of supplementary views per section—even when additional items are added or removed from that section. This is evidenced by the fact that when you invoke -[ASCollectionView insertItemsAtIndexPaths:], the data source method -[ASCollectionDataSource collectionView:nodeForSupplementaryElementOfKind:atIndexPath:] is not invoked, preventing consumers from specifying a new number of supplementary nodes for the new set of items.
With this change, the set of supplementary nodes for a section is now recalculated not only on section-level mutations, but also on item-level mutations as well. This adds item-level counterparts to the section-level `-prepareFor...` subclassing hooks in `ASDataController+Subclasses.h` to make this possible.
This should fix#1278 and #1322
This has been tested in my project and seen to fix the assertion. Open to suggestions on how to test in a more universal way.
fc5eef3269