Fix handling of serialized attributes

A serialized attribute value in @attributes is actually wrapped in
another class since AR 3.2.0. This was not respected during deletion.

Fixes "undefined method `unserialized_value'" after deleting a
permanent record and then trying to access a serialized attribute
on the same model instance.

I'm only half happy with this solution (manually re-wrapping the attributes), 
so if you can think of something better... ;)