Fix for polymorphic association field selector not working with AR

AR stores and queries the STI base class in polymorphic associations. This commit correctly sets the <polymorphic>_type field to the object's STI base class (not the object's class).

Note: I used the rubygem (0.6.5) version because master (0.6.6) is broken. The cerulean theme has not been updated and gives an error, plus a few other css issues., however you should be able to pull also on top of master