webdav: Allow to save documents again, after locking is implemented.

We need to extend PUT method with lock token in 'If' header, so that it
does not fail on a resource locked by us.

Change-Id: I82a9e3303a34607dba8bb874d1eb0fac23b9f11d
(cherry picked from commit e250a013bae4b58a9304518aefc6da0dab574103)