conditional validation not working in models that inherit and set collection

[cpmurphy]

A model should be able to add a conditional validation to a key defined in a parent model, even if it uses its own collection. This was working in mongomapper 0.11.1 but it doesn't work in master currently. Instead the conditional on the validation is apparently ignored, i.e. the validation is applied when it shouldn't be.

Here's a test case:

    # add this to spec/functional/validations_spec.rb
    context "conditional validation" do
      it "should do conditional validation in child classes" do
        parent_class = Doc do
          key :name, String
        end
        class ChildDoc < parent_class
          validate :never_valid, :if => lambda { false }
          set_collection_name 'anything'

          def never_valid
            errors.add(:name, 'not valid, sorry')
          end
        end
        ChildDoc.new.should be_valid
      end
    end