dockerflow.logging.JsonLogFormatter should handle dates better

[diox]

#8 fixes the exception that can happen when you log objects. What'd be even nicer, as @jezdez pointed out, would be to handle datetimes/dates : currently they are serialized as "datetime.datetime(2017, 4, 6, 9, 27, 7, 479427)" because that's the repr() for a datetime object, but we could do better by using DjangoJSONEncoder, which knows how to handle those objects and more.

Something like this should work, to test:

class SafeJSONEncoder(json.JSONEncoder):
    def default(self, o):
        return repr(o)

class SafeDjangoJSONEncoder(DjangoJSONEncoder, SafeJSONEncoder):
    pass

Then use cls=SafeDjangoJSONEncoder when doing the json.dumps.

[diox]

One problem with that approach: at the moment src/dockerflow/logging.py is generic, does not depend on django. Not sure how to refactor that in a clean way.

[jezdez]

@diox Oh, we'd have to ship an own encoder, literally just copy/pasting from Django. I think I've seen similar encoders in non-Django projects, so no huge problem.