Merge pull request #585 in ZP/z-push from bugfix/ZP-1278-caldav-backend-does-not-pick-up-user to develop

* commit 'c6406d4a':
  ZP-1278 Fix caldav backend so the user and (if supplied) domain are picked up for blank organizer fields when sent to mobile. Released under the Affero GNU General Public License (AGPL) version 3.