You're right the force ssl function has a bug. In the file components/com_hikashop/controllers/checkout.php near line 115, you should have the line
if($config->get('forceSSL',0) && (empty($_SERVER['HTTPS']) OR strtolower($_SERVER['HTTPS']) != "on")){
If you replace it by the line
if($config->get('force_ssl',0) && (empty($_SERVER['HTTPS']) OR strtolower($_SERVER['HTTPS']) != "on")){
It should force the HTTPS during the checkout.
However, please note that it won't have any effect on the fact that you will be redirected to Authorize.net for the payment. Authorize.net provides two APIs to integrate with them: SIM and AIM. We chose to integrate first with the SIM interface which do not require you to have SSL active but redirects the customer to Authorize.net for the payment. The AIM interface enables you to capture credit card information directly on your website but our plugin does not yet integrates with it.