Generate a copy of json_fields before adding message  #652

Description

We had a problem where we wanted to log a payload to another service via "json_fields", however, the logging handler added the key "message" to the payload so we started having errors.

payload = {"id": 1}
logging.info("hello", extra={"json_fields": payload})
# payload becomes {"id": 1, "message": "hello"}

Proposed solution

Make a copy of the "json_fields" param inside extra before processing it.