Backport of upstream patch to fix errors with spaces in usernames

https://github.com/fruux/sabre-dav/issues/365
https://github.com/owncloud/contacts/issues/137#issuecomment-25155120
https://github.com/owncloud/calendar/issues/154
Add missing class name to method call
Merge pull request #53 from owncloud/sabredav-spaces-in-usernames

Backport of upstream patch to fix errors with spaces in usernames