Payment gateway selection form might be displayed to customers

In the event that your default payment gateway is disabled or uninstalled and you switch to another one, the payment gateway selection form might be displayed to your customers during checkout if you don't go back to the payment method form and save it with a new default payment gateway. I think the place to fix this is to use the first payment gateway as the default one if $default is specified in the parameters to uc_payment_process() but the default gateway no longer exists. We definitely do not want the current behavior, but it's far from a showstopper since all you have to do is resubmit the payment methods form to fix it.