Cleanup OUString-use in filter/html

Change-Id: If2749e604d0375fa0af0eac9244c533e177f09ab
Reviewed-on: https://gerrit.libreoffice.org/10599
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>