Merge r593816 from trunk:

* Generate valid XHTML output by adding the xhtml namespace.

PR: 43649
Submitted by: Jose Kahan <jose w3.org>
Reviewed by: rpluem

Submitted by: rpluem
Reviewed by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@596675 13f79535-47bb-0310-9956-ffa450edef68