mod_dav: Do not fail PROPPATCH when prop namespace is not known.

PR: 52559
Backports: r1476644
Submitted by: Diego Santa Cruz <diego.santaCruz spinetix.com>
Reviewed by: minfrin, wrowe, rjung



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1497457 13f79535-47bb-0310-9956-ffa450edef68