Support @Spy AbstractClass/Interface

Had to fix a few existing tests because ```spy(Class)``` doesn't like private ocnstructors.

I think the most common case would be for a test to ```@Spy PrivateNestedClass```. By default, private class's default constructor is private so these tests were implicitly calling the private constructor.

Another option would be to check and skip calling constructor for private constructors. Although, that would be behavior-changing too.
well spotted. usingConstructor does not necessarily make an object a spy but I think this approximation is good enough. Might be worth increase coverage for MockingDetails API