RZCollectionTableView skips section headers/footers

`RZCollectionTableView` has a bug. On first reload, as soon as it encounters a section without header and footer (i.e. header/footer height is 0), any subsequent section footers and headers are not loaded (although their height is calculated correctly).

I have modified the demo app to demonstrate this problem:
https://github.com/deville/RZCollectionTableView
