Fix inner/outer subject confusion when using a named subject.

Fixes #800.

Note that the complexity of supporting `super` in a named subject makes me think that we should disable `super` in named subjects altogether instead.
Improve on e93b5d0abae49787837189fc248f30de392886ff.

- Ensure the NamedSubject override module gets inserted in the
  ancestor chain at the right place (this wasn't guaranteed before).
- Raise an error when `super` is used in a named subject. I don't
  think we should support it.
Merge pull request #804 from rspec/issue-800

Candidate fix for #800