Fix changing config in subclass having global effect

lib/active_model/serializer.rb

@@ -192,6 +192,7 @@ def self.inherited(base)
       base._attributes_data = _attributes_data.dup
       base._reflections = _reflections.dup
       base._links = _links.dup
+      base.config = ActiveSupport::InheritableOptions.new(config)
     end
 
     # @return [Array<Symbol>] Key names of declared attributes

test/serializers/configuration_test.rb

@@ -5,6 +5,10 @@
 module ActiveModel
   class Serializer
     class ConfigurationTest < ActiveSupport::TestCase
+      class CustomSerializer < ActiveModel::Serializer
+        config.adapter = :json_api
+      end
+
       def test_collection_serializer
         assert_equal ActiveModel::Serializer::CollectionSerializer, ActiveModelSerializers.config.collection_serializer
       end
@@ -29,6 +33,10 @@ def test_setting_array_serializer_sets_collection_serializer
       def test_default_adapter
         assert_equal :attributes, ActiveModelSerializers.config.adapter
       end
+
+      def test_subclass_adapter
+        assert_equal :json_api, CustomSerializer.config.adapter
+      end
     end
   end
 end