html export: Fix export of checked checkbox and radiobutton.

Change-Id: Ia77f9840df7508d96292aac63e65714a7892342f
Reviewed-on: https://gerrit.libreoffice.org/12049
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>