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.
Merge pull request #30 from emjot/ar32-serialized-attributes

Fix handling of serialized attributes