This helps in case the HTML filter is given an XHTML that has an
explicit namespace alias for <http://www.w3.org/1999/xhtml>.

Change-Id: I437fa85ba19ce907c9c4b10c8d10aaf2217dc0ea
Reviewed-on: https://gerrit.libreoffice.org/50380Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>