Check whether remote DAV server accepted the mtime on touch

ownCloud as remote DAV always accepts the mtime on touch, but other
servers like Apache's DAV server doesn't. The latter doesn't give any
visible hint in its response to detect this case, so this fix does a
subsequent PROPFIND to check whether the mtime was actually set.

Since a touch() operation seldom happens (only on uploads), the minor
performance loss should hopefully be acceptable.