Package webdav requires the Go standard library's encoding/xml package version
1.5 or greater.

Fixes #10904

Change-Id: Idf2915e581f4efa6f00e7701785ea258f9e1a8f4
Reviewed-on: https://go-review.googlesource.com/10243Reviewed-by: Robert Stepanek <robert.stepanek@gmail.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>