json encode datetime as iso8601 string when returned in REST body.

This ensures that fields stored in the DB as mongo native dates then bson decoded into python *datetime* in query results will be json encoded as iso8601 strings instead of ```{"$date": <int>}``` when returned in the REST API body.
Can this be named something that isn't so generic. Perhaps `datetime_to_iso8601_json_encoder` or `custom_json_encoder`