Rebased onto 1.1.x
Merge pull request #442 from mpdude/fix-clear-filters-on-collection-1.1.x

AssetCollection::clearFilters() does not work once the collection has been iterated (1.1.x)
Add a testcase to demonstrate #427
Test case for the "additional question 1" in #427
Perform check at leaf assets to make sure all filters are considered.

The collection's filters will be propagated to the leaves by the AssetCollection.
Use the interface, of course