Displayname WebDAV property should be XML escaped. With current
implementation a file with name '<.txt' would make the XML
invalid.

Fixes golang/go#17158

Change-Id: Ib3b5376094edc957ed15adf511bd1050ea13d27e
Reviewed-on: https://go-review.googlesource.com/29297Reviewed-by: Nigel Tao <nigeltao@golang.org>