Fixing incorrect method signature for msg_att

Otherwise, it crashes:

```
gmail = Gmail.connect(login, password)
eml = gmail.mailbox(folder_name).unread.first
 x = eml.attachments
```

was causing crash on wrong number of arguments.
[@johnnyshields commented](https://github.com/bootstraponline/gmail/commit/52f60b700e8609d9a6c6cfcd5daecdae49075bfd#commitcomment-8828593) about supporting Ruby 1.9.

> Should this should check for arity on the different Ruby versions? We need to continue to support Ruby 1.9.3 for the time being.