polymorphic assoc with a sti class issue fixed

http://stackoverflow.com/questions/9628610/why-polymorphic-association-doesnt-work-for-sti-if-type-column-of-the-polymorph
Merge pull request #23 from nando/polymorphic_with_sti_issue_fixed

Awesome! Thank you for polymorphic assoc with a sti class issue fixes. @nando