Bridging stickers (and maybe other documents) is broken

[tulir]

[2019-02-14 22:25:09,790] [[email protected]] Failed to handle Telegram update
Traceback (most recent call last):
  File "mautrix_telegram/abstract_user.py", line 142, in _update_catch
    await self._update(update)
  File "mautrix_telegram/abstract_user.py", line 195, in _update
    await self.update_message(update)
  File "mautrix_telegram/abstract_user.py", line 384, in update_message
    return await portal.handle_telegram_message(self, sender, update)
  File "mautrix_telegram/portal.py", line 1627, in handle_telegram_message
    relates_to=formatter.telegram_reply_to_matrix(evt, source))
  File "mautrix_telegram/portal.py", line 1397, in handle_telegram_document
    document.thumb, is_sticker=attrs["is_sticker"])
AttributeError: 'Document' object has no attribute 'thumb'