Method utils.jwt_payload_handler ignores USERNAME_FIELD

[rm--]

I have seen with #18 the serializers.py got an username access with paying attention to USERNAME_FIELD. But in utils.jwt_payload_handler it is still missing. (;

In my option there should be an implementation like in django-compat, also for compatibility reasons.

[hoIIer]

@rm-- I agree the django-compat snippet looks good, could you submit a PR with the changes?

[rm--]

@erichonkanen Of course. Give my some days.

[rm--]

thanks!