Correct stub undefined parent modules all the way down when stubbing a nested constant.

Created by: xaviershay

This is necessary for verifying doubles to work correctly, since they need to distinguish between real and stubbed constants.

This fell out quite nicely. I'm not confident I've either named the Constant spec correctly nor covered the correct cases (though it certainly fails before this change), so please review that in particular.

Fixes #529 (closed).

R @myronmarston @JonRowe