Revert "ActiveRecord::Base#<=> has been removed.  Primary keys may not be in order," -- will be replaced with a check to ensure that the keys used for comparison are integers, and only fail if they are not.

This reverts commit 6256734e2d0bdd89f4b5d11da259d40afa0c95c7.

Conflicts:
	activerecord/CHANGELOG.md
Add test verifying that you can only sort ARs of the same type