`ujson` does not support `.dumps(cls=SomeEncoder)` `cls` parameter

[EivV]

As per ultrajson/ultrajson#124

If you have a package that requires ujson, it is automatically picked up by the compat.py and used in JSONFormatter thereafter. Unfortunately, ujson is not fully compatible with the built-in json.dump and it does not understand the cls parameter.

[albertomr86]

Hello.

After being off for a good amount of days, I'm planning to start doing clean up of issues.

Meanwhile, if you have some working code, please, add an MR, and I will be happy to review it.

Thanks.