WebDAV clients can't sort folders by date because 'getlastmodified' live
property is not set.

Fixes golang/go#15334.

Change-Id: Ie56fcf8ae98173878e0972c0f9401151953334ff
Reviewed-on: https://go-review.googlesource.com/23422Reviewed-by: Nigel Tao <nigeltao@golang.org>