Refactoring KIMAP::LoginJob::handleResponse() method for better correctness.

Besides better readability, this makes handleResponse() method
handle not only untagged response caused by CAPABILITY command,
but also unsolicited untagged CAPABILITY response caused by AUTHENTICATE
command which is returned by Dovecot IMAP server for compatibility with
MS Outlook.
This fixes bug #249992.

BUG: 249992
REVIEW: 103854