User-provided dumps to build swagger_dict

[letrec]

It would be great if it was possible to pass a dumps function to setup_aiohttp_apispec that should be used in https://github.com/maximdanilchenko/aiohttp-apispec/blob/master/aiohttp_apispec/aiohttp_apispec.py#L97 to serialize swagger dict. That would enable usage of types in parameter defaults and request schema examples that the default serializer is not able to handle.