Always send EmptyDataSet view to back

This supports collection views or scroll views implementing their own static non-scrolling content, like our app.

I found when this was changed but am unsure of the intent:
https://github.com/dzenbot/DZNEmptyDataSet/commit/4e376e42fca8cd82a88877af26624d8474240418

If this looks good as-is then the PR is ready to go, otherwise know that I am having an issue where without this change my collectionview's static header is getting covered over. I imagine anyone with custom static non-scrolling content using this library will have the same problem.