Define attrs in a module when building AM models in tests

When a developer builds an ActiveModel model and wants to override an
attribute, she may do so using `super` to call the original method. For
instance:

    def foo=(value)
      super(value.next)
    end

as opposed to:

    def foo=(value)
      @foo = value.next
    end

spec/support/unit/helpers/model_builder.rb

@@ -34,8 +34,15 @@ module UnitTests
     def define_active_model_class(class_name, options = {}, &block)
       accessors = options.fetch(:accessors, [])
 
+      attributes_module = Module.new do
+        accessors.each do |column|
+          attr_accessor column.to_sym
+        end
+      end
+
       define_class(class_name) do
         include ActiveModel::Validations
+        include attributes_module
 
         def initialize(attributes = {})
           attributes.each do |name, value|
@@ -43,10 +50,6 @@ module UnitTests
           end
         end
 
-        accessors.each do |column|
-          attr_accessor column.to_sym
-        end
-
         if block_given?
           class_eval(&block)
         end