Add support of marshmallow schema instance as nested parameter.

[AndreyKis]

As seen from this answer, marshmallow supports usage of schema instance as nested field of outer schema.

Question in that StackOverflow thread contains one of the examples, when this could be useful and exact use-case we have in our project.

Unfortunately, flask_accepts doesn't support such behaviour at the moment. Usage of schema instance results in Error, thrown in flask_accepts/utils.py at this line of code:

I could submit a fix for that by the end of today. The only change added will be this one:

Note: In the future this could be improved to iteration over type_map keys and ensuring isinstance for all of them.
This would make available usage of marshmallow feature to extend existing marshmallow fields

[apryor6]

👍 this would be a good feature. Open a PR w/ relevant tests and I'm happy to accept

[AndreyKis]

Great, will do, thank you