[PYTHON] mapped attributes parameter for to_dict

Created by: rienafairefr

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Description of the PR

I found myself trying to compare a model representation with its representation on the API side, and the to_dict method was returning the model dict with the modified (snake_case) attributes instead of the original attributes that the API internally used. I added a mapped_attributes argument to the to_dict method, Which means:

> # incoming API JSON representation
> incoming = {"BasicProperty": "stuff"}
> # python model (if model casing is set to snakecase)
> model = {"basic_property": "stuff"}
> model.to_dict()
{"basic_property": "stuff"}
> model.to_dict(mapped_attributes=True)
{"BasicProperty": "stuff"}
> assert incoming == model.to_dict(mapped_attributes)

Seems backward compatible to me, although it might fail if a model has an attribute value's class that implements a to_dict method without the mapped_attributes parameter (mixed-version generation maybe ? )