self.message_id is not callable (str object)

```
Traceback (most recent call last):
File "./bot.py", line 777, in
main(fingerprint)
File "./bot.py", line 725, in main
messages = fetcher.get_all_mail()
File "./bot.py", line 310, in get_all_mail
return self.get_imap_mail()
File "./bot.py", line 299, in get_imap_mail
message_id=message_id))
File "./bot.py", line 531, in __init__
self._parse_for_openpgp()
File "./bot.py", line 583, in _parse_for_openpgp
logging.warning("More than one signed part in this message. That's weird... {0}".format(self.message_id()))
TypeError: 'str' object is not callable
```