Port of lexrupy's fix for rails 3.2 for oracle

I've ported lexrupy's pull request #167 with the fix for the "unserializable_attribute?" error for oracle.

Sadly i wasn't able to run the tests as i don't use it within a rails application. I use activerecord as a standalone plugin for one of my jruby projects but i've tested it with my real world application and got rid of the error message mentioned above.