Previously `has_one` and `has_many` associations were using the
`one` and `many` keys respectively. Both of these keys have special
meaning in I18n (they are considered to be pluralizations) so by
renaming them to `has_one` and `has_many` we make the messages more
explicit and most importantly they don't clash with linguistical
systems that need to validate translation keys (and their
pluralizations).

The `:'restrict_dependent_destroy.one'` key should be replaced with
`:'restrict_dependent_destroy.has_one'`, and
`:'restrict_dependent_destroy.many'` with
`:'restrict_dependent_destroy.has_many'`.

[Roque Pinel & Christopher Dell]