Skip response fields where value is None

[mjreiss]

Hi,

It would be nice if there was a way to filter out any fields where the return value is None. Flask-RESTX provides this functionality in their marshal_with decorator, which allows a skip_none parameter to be passed in (docs). The solution for this would be to allow the responds decorator to similarly take a skip_none bool parameter (defaulting to False), which would filter out any None value fields produced in the serialized object.

I can try and get a tested solution put into a PR. I think it should just be something like this in def responds:

if skip_none:
	serialized = {k:v for k, v in serialized.items() if v is not None}