Add before_marshmallow hook to POST handler
Add before_marshmallow hook to POST handler

When this hook was introduced by @kumy  in https://github.com/miLibris/flask-rest-jsonapi/pull/123, it was not added to the `post()` method. I'm pretty sure this is an oversight, as it's just as useful there as anywhere else, and there's no discussion about specifically omitting it.
miLibris/flask-rest-jsonapi