fixes :test option passed to ActiveMerchant::Billing::AuthorizeNetGateway to reflect server setting

Well the preference server still needs to be added for this payment method it's only been added to non cim gateway.

This looks to me like not having 'live' as the value for the new server preference that has no default will result in the preferences[:test] being set to true, which I thought it was what decides whether to hit the test or production server.