Follow up to bbe7fe41 to fix enum leakage across classes.

The original attempt didn't really fix the problem and wasn't testing the
problematic area. This commit corrected those issues in the original commit.

Also removed the private `enum_mapping_for` method. As `defined_enums` is now a
method, this method doesn't provide much value anymore.

Conflicts:
	activerecord/lib/active_record/validations/uniqueness.rb