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

This only has an effect when HtmlWriter is used to write elements,
though.

Change-Id: I161caf4bb6bc6d0c21a7a67433b08bb060b447f1
Reviewed-on: https://gerrit.libreoffice.org/50448Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>