Restoring polymorphic has_one relationships errored because paranoia was
not correctly looking up the foreign_key.

Output from failing test -

ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: parent_model_id: SELECT  "polymorphic_models".* FROM "polymorphic_models" WHERE ("polymorphic_models"."deleted_at" IS NOT NULL) AND (parent_model_id)  ORDER BY "polymorphic_models"."id" ASC LIMIT 1

The test sets up a PolymorphicModel, which has_many parents. The
ParentModel then has a has_one relationship with PolymorphicModel. When
restoring, the foreign key is set as - `self.class.name.to_s.underscore_id`
which will be parent_model_id, instead of the :as option.