Clean up I18n stored translations after test.
Deprecate `serialized_attributes` without replacement

We've stopped using it internally, in favor of polymorphism. So should
you!
Merge pull request #15715 from zuhao/refactor_actionview_cleanup_I18n_translations

Clean up I18n stored translations after test.
Merge pull request #15704 from sgrif/sg-deprecate-serialized

Deprecate `serialized_attributes` without replacement
Don't save through records twice

If the through record gets created in an `after_create` hook that is
defined before the association is defined (therefore after its
`after_create` hook) get saved twice. This ensures that the through
records are created only once, regardless of the order of the hooks.