Fix 'undelete-mailbox' issue for undeleting all user's mailboxes

Summary: if 'hextimestamp' key does not exists in _mailfolder dictionary, pykolab throughs traceback.

Test Plan: None

Reviewers: vanmeeuwen, machniak

Reviewed By: machniak

Subscribers: petersen

Differential Revision: https://git.kolab.org/D605