Fix for lack of variance in .NET 3.5.
Fix for lack of variance in .NET 3.5.

andrewdavey/cassette,honestegg/cassette,damiensawyer/cassette,andrewdavey/cassette,BluewireTechnologies/cassette,BluewireTechnologies/cassette,damiensawyer/cassette,damiensawyer/cassette,honestegg/cassette,honestegg/cassette,andrewdavey/cassette