This is an alternative of #27714.

If `has_one :through` association has set `nil`, `through_record` is
destroyed but still remain loaded target in `through_proxy` until
`reload` or `reset` explicitly.

If `through_proxy` is not reset (remain destroyed (frozen) target),
setting new record causes `RuntimeError: Can't modify frozen hash`.

To prevent `RuntimeError`, should reload `through_record` that has been
destroyed in `create_through_record`.