Execute the given Proc condition in the context of the object to allow for dynamic conditions

Hi,
currently the Proc specified in conditions is evaluated in the context of the FormBuilder. it should be evaluated in the context of the object so that conditions that are dependant on the object state can be specified (as it works in active record)

the test example only shows the one tag that has the same id that the user id. a practical example would be if a User and Tag would belongs_to a Group and one would want to scope the show only the Tags from the same group to the user.