StripPrefix in the webdav package strips prefixes from requests
(including the Destination headers) but cannot handle the paths
in the xml entities responses which are confusing some clients
(e.g. cadaver).

This patch replaces StripPrefix with Prefix in the Handler to
handle prefixes both in the requests and in the xml entities
responses.

Change-Id: I67062e30337b2ae422c82a2f927454f5a8a00e34
Reviewed-on: https://go-review.googlesource.com/13857Reviewed-by: Nigel Tao <nigeltao@golang.org>