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.
This indicates that the PR is a breaking change :) We cannot do breaking changes before 2.0 I cannot really merge this PR at this stage.