<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def mutate_json_record(self, json_record):
 """Override it to convert fields of `json_record` to needed types. Default implementation converts `datetime` to string in ISO8601 format. """

        for attr_name in json_record:
            attr = json_record[attr_name]
            if isinstance(attr, datetime):
                json_record[attr_name] = attr.isoformat()
        return json_record