Support passing a custom ArraySerializer for has_many associations

Thanks @phoet

Closes #316

test/unit/active_model/serializer/has_many_test.rb

@@ -129,6 +129,21 @@ module ActiveModel
           comments: [{ content: 'fake' }, { content: 'fake' }]
         }, @post_serializer.as_json)
       end
+
+      def test_associations_using_a_given_array_serializer
+        @association.embed = :ids
+        @association.embed_in_root = true
+        @association.serializer_class = Class.new(ActiveModel::ArraySerializer) do
+          def serializable_object
+            { my_content: ['fake'] }
+          end
+        end
+
+        assert_equal({
+          'post' => { title: 'Title 1', body: 'Body 1', 'comment_ids' => @post.comments.map { |c| c.object_id } },
+          comments: { my_content: ['fake'] }
+        }, @post_serializer.as_json)
+      end
     end
   end
 end