net/webdav: declare namespaces used in lock owner XML

[hacdias]

The content of a LOCK request was captured verbatim with
innerxml and sent back into the response, which only declares the DAV:
namespace. Prefixes declared on an ancestor element, such as the
root, were left undefined, producing malformed XML.

Decode through xmlValue, as PROPPATCH does for property values,
so the encoder redeclares every namespace in use.

Fixes golang/go#81773

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