Add specs for mass-assignment security.
user
```ruby
require 'spec_helper'
require 'dm-rails/mass_assignment_security'

# Because mass-assignment security is based on ActiveModel we just have to
# ensure that ActiveModel is called.
describe DataMapper::MassAssignmentSecurity do
  before :all do
    class Fake
      super_module = Module.new do
        def _super_attributes=(*args)
        end

        def attributes=(*args)
          self._super_attributes = *args
        end
      end
      include super_module

      include ::DataMapper::MassAssignmentSecurity
    end
  end

  describe '#attributes=' do
    it 'calls super with sanitized attributes' do
      attributes = { :name => 'John', :is_admin => true }
      sanitized_attributes = { :name => 'John' }
      model = Fake.new
      model.should_receive(:sanitize_for_mass_assignment).with(attributes).and_return(sanitized_attributes)
      model.should_receive(:_super_attributes=).with(sanitized_attributes)

      model.attributes = attributes
    end

    it 'skips sanitation when called with true' do
      attributes = { :name => 'John', :is_admin => true }
      sanitized_attributes = { :name => 'John' }
      model = Fake.new
      model.should_receive(:_super_attributes=).with(attributes)

      model.send(:attributes=, attributes, true)
    end
  end
end
```
assistant