webdav: advertise PUT for existing files

Change-Id: I285f93e562ca0c6bdc6168e6c47b28c7caef7cf5
Reviewed-on: https://go-review.googlesource.com/3501
Reviewed-by: Nick Cooper <nmvc@google.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
1 file changed