Follows inheritance with a namespace

Co-authored-by: Rafael Gaspar <rafael.gaspar@me.com>
Co-authored-by: Darryl Pogue <darryl@dpogue.ca>
Co-authored-by: Artin Boghosian <artinboghosian@gmail.com>

test/serializers/serializer_for_with_namespace_test.rb

@@ -9,6 +9,7 @@ module ActiveModel
         attributes :title, :author_name
         associations :publisher, :pages
       end
+      class Ebook < Book; end
       class Page < ::Model; attributes :number, :text end
       class Publisher < ::Model; attributes :name end
 
@@ -85,6 +86,11 @@ module ActiveModel
         }
         assert_equal expected, result
       end
+
+      test 'follows inheritance with a namespace' do
+        serializer = ActiveModel::Serializer.serializer_for(Ebook.new, namespace: Api::V3)
+        assert_equal Api::V3::BookSerializer, serializer
+      end
     end
   end
 end