Update #msg_att for ruby v2.0 compatibility

Please update #msg_att in Net::IMAP::ResponseParser for ruby v2.0 compatibility as suggested by:
https://github.com/awakia/gmail/commit/cac1f29a0d96ff755c5487ae20c7272aafc292af

as it raises the error:
ArgumentError: wrong number of arguments (1 for 0) when fetching emails.