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.
I assume this still means that you'd prefer it to be added in v2.0, not earlier, right?

Just want to make sure I'm not dropping the ball here.

(Btw, when is v2.0 expected to release?)