Return serialized version of record.INFO

def _serialize_info(self, record):
        """Return serialized version of record.INFO"""
        result = []
        for key, value in record.INFO.items():
            info = self.header.get_info_field_info(key)
            if info.type == "Flag":
                result.append(key)
            else:
                result.append("{}={}".format(key, format_value(info, value, "INFO")))
        return ";".join(result)