fixes the following scenarios:
* I have validates_inclusion_of on the type field for a polymorphic belongs_to association. I assign a new record to the model's polymorphic relationship of the proper type. validation fails because the type field has not been updated.
* I replace the value for a ppolymorphic association to a new record of another class. The type field still says its the previous class, and the id field points to the previous record as well.
[#191 state:closed]