webdav: have copyFiles copy dead properties.

Change-Id: I778e26c06f153fb705b450052d3721fec9a3082b
Reviewed-on: https://go-review.googlesource.com/10471
Reviewed-by: Robert Stepanek <robert.stepanek@gmail.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed