Create DefaultSerializer so that as_json uses same interface.

This is to ensure that PORO's as_json is called if no serializer
is specified.

Original behaviour was that serializable_hash was being called,
overriding the as_json method.

test/array_serializer_test.rb

@@ -12,7 +12,7 @@ class ArraySerializerTest < ActiveModel::TestCase
     serializer = array.active_model_serializer.new(array, :scope => {:scope => true})
     assert_equal([
       { :model => "Model" },
-      { :last_name => "Valim", :ok => true, :first_name => "Jose", :scope => true },
+      { :first_name => "Jose", :last_name => "Valim", :ok => true, :scope => true },
       { :title => "Comment1" }
     ], serializer.as_json)
   end
@@ -51,5 +51,4 @@ class ArraySerializerTest < ActiveModel::TestCase
       { :title => "Post2" }
     ], serializer.as_json)
   end
-
 end

test/test_fakes.rb

@@ -68,7 +68,7 @@ class MyUserSerializer < ActiveModel::Serializer
   end
 end
 
-class CommentSerializer
+class CommentSerializer < ActiveModel::Serializer
   def initialize(comment, options={})
     @object = comment
   end