rename simple adapter to json

test/serializers/adapter_for_test.rb

@@ -11,7 +11,7 @@ module ActiveModel
 
       def test_returns_default_adapter
         adapter = ActiveModel::Serializer.adapter
-        assert_equal ActiveModel::Serializer::Adapter::SimpleAdapter, adapter
+        assert_equal ActiveModel::Serializer::Adapter::JsonAdapter, adapter
       end
 
       def test_overwrite_adapter_with_symbol

test/serializers/configuration_test.rb

@@ -8,7 +8,7 @@ module ActiveModel
       end
 
       def test_adapter
-        assert_equal :simple, ActiveModel::Serializer.config.adapter
+        assert_equal :json, ActiveModel::Serializer.config.adapter
       end
     end
   end