Define attribute methods in a module included in the base class

This will make possible to call `super` in method overrides and to
define the methods after the `attributes` method was called without
warnings.

lib/active_model/serializer.rb

@@ -19,6 +19,10 @@ module ActiveModel
         base._associations = (_associations || {}).dup
       end
 
+      def generated_modules
+        @generated_attribute_methods ||= Module.new.tap { |mod| include mod }
+      end
+
       def setup
         @mutex.synchronize do
           yield CONFIG
@@ -90,9 +94,9 @@ end
 
           @_attributes << striped_attr
 
-          define_method striped_attr do
+          generated_modules.define_method striped_attr do
             object.read_attribute_for_serialization attr
-          end unless method_defined?(attr)
+          end
         end
       end
 
@@ -130,9 +134,9 @@ end
         options = attrs.extract_options!
 
         attrs.each do |attr|
-          define_method attr do
+          generated_modules.define_method attr do
             object.send attr
-          end unless method_defined?(attr)
+          end
 
           @_associations[attr] = klass.new(attr, options)
         end

test/fixtures/poro.rb

@@ -128,12 +128,12 @@ class UserInfoSerializer < ActiveModel::Serializer
 end
 
 class ProfileSerializer < ActiveModel::Serializer
+  attributes :name, :description
+
   def description
-    description = object.read_attribute_for_serialization(:description)
+    description = super
     scope ? "#{description} - #{scope}" : description
   end
-
-  attributes :name, :description
 end
 
 class CategorySerializer < ActiveModel::Serializer
@@ -147,7 +147,7 @@ class PostSerializer < ActiveModel::Serializer
 
   def title
     keyword = serialization_options[:highlight_keyword]
-    title = object.read_attribute_for_serialization(:title)
+    title = super
     title = title.gsub(keyword,"'#{keyword}'") if keyword
     title
   end