refs https://github.com/TryGhost/Ghost/issues/12716
- The code in serializePostModel was broken and always defaulted to 'v3'! It refered to non-existent `model.get('api_version')` there's no such field in posts model! Changed the implementation so that the API version is passed in as a parameter to the method instead
- The style of providing "defaults" everywhere creates a need for future maintenance when we bump the version e.g in Ghost v5. Maybe reworking these methods to require a passed version and throwing an error instead would be more maintainable long-term?
