Fix serialization bug

src/videoag_common/api_object/fields/relationship_fields.py

src/videoag_common/api_object/fields/special_fields.py

src/videoag_common/api_object/object.py

src/videoag_common/api_object/object_class.py

src/videoag_common/objects/course.py

src/videoag_common/objects/site.py