code: def mutate_json_record(self, json_record):
          
          for attr_name in json_record:
              attr = json_record[attr_name]
              if isinstance(attr, datetime):
                  json_record[attr_name] = attr.isoformat()
          return json_record
docs: Override it to convert fields of `json_record` to needed types.
  
          Default implementation converts `datetime` to string in ISO8601 format.