Revert deleted tests by past commit

Hi, in this commit:

https://github.com/rails/rails/commit/080345baca1076a9788dff4803153...

some code was refactored and the tests deleted. I think the refactor was great, but the tests shouldn't have been deleted, as the validations still applies. This tests should be keep so the functionality is guaranteed independent of the implementation.

I attach a patch reverting only the tests part.