dirty.rb

    def reset_attribute!(name)
      value = attribute_was(name)
      @attributes[name] = value
      modifications.delete(name)
    end

dirty_spec.rb

# ...
# this is line 396
      context "change previously unset attribute" do
        before do
          @person.ssn = "111-22-3456"
        end

        it "resets the value to unset" do
          @person.reset_attribute!("ssn")
          @person.ssn.should be_nil
        end

        it "removes the field from the changes" do
          @person.reset_attribute!("ssn")
          @person.changed.should == []
        end
      end
# ...