Prevent an inner group method def from overriding an outer group subject

This is something I recently realized was unspecified by our specs.  I think the behavior we had it before is actually confusing and should be considered a bug, so I changed it...but there's always the chance that there are spec suites out in the wild that rely on the old behavior -- so I'd like another pair of eyes (/cc @alindeman and @dchelimsky...) on this before I merge.