make msgpack encoder compliant with JSON one

[sevdog]

As of now kombo default JSON encoder supports a type-conversion mechanism

https://github.com/celery/kombu/blob/241b5dcff8a7c8ad411e1b325d59e47acfa9e1ed/kombu/utils/json.py#L36-L58

Also msgpack.packb supports a default callback to handle non-native-types. If msgpack serializer behaves just like JSON one than it would not be a problem to switch between one of these two serializers.

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