Fix imap extensions in ruby 1.9.3

These constants are defined in Net::IMAP::ResponseParser and if you
don't put the full path to them here it won't work on ruby 1.9.3.
Update depcrecated RSpec syntax.

DEPRECATION: `expect { }.not_to raise_error(SpecificErrorClass)` is
deprecated. Use `expect { }.not_to raise_error` (with no args) instead.