webdav: if TestReadProppatch fails, print InnerXML as a string.

A string is more readable than the default representation of a []byte.

Change-Id: I107c25aa09798df7c7766847beef4de124f44006
Reviewed-on: https://go-review.googlesource.com/10851
Reviewed-by: David Symonds <dsymonds@golang.org>
Reviewed-by: Robert Stepanek <robert.stepanek@gmail.com>