Add test for Cyrus imap proxy bug

Summary:
Add test for Cyrus imap proxy bug
Update rcube_imap_generic from Roundcube master. It includes fixed error handling on FETCH.

Test Plan: ./phpunit

Reviewers: #hkccp_developers, mollekopf

Reviewed By: #hkccp_developers, mollekopf

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D4637