This allows requesting text/html, with falling back to plain text if
necessary.

Change-Id: Ie6d4e0e173311ba018553043b6a869abf193bf6f
Reviewed-on: https://gerrit.libreoffice.org/16377Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>