docs: document filter_overrides

Found it while i was going to reimplement it :)
Fix typo in FilterView documentation

It's filtered_class and not filter_class. Fixes #135
Merge pull request #151 from xrmx/viewstypo

Fix typo in FilterView documentation
Merge pull request #150 from xrmx/overrides

docs: document filter_overrides
Fixed warnings in the testsuite.
Merge pull request #154 from apollo13/tests

Fixed warnings in the testsuite.