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

Also server is still a string, stored as a string preference, nothing has change about that. The meaning of the preference is corrected in this pull. 

Backwards migration wouldn't assist because anyone using a sandbox credential must be hitting the test environment anyway, this bug makes it so that if you have test mode OFF (in the Spree interface & preference), you hit the Authorize.net production server, which returns a response telling you that your merchant ID is invalid.

I don't think a backwards migration makes sense. the only affect group for 'surprise' change in behavior is someone who currently has Test Mode ON with a setting of 'live' in the server field (which I bet is a minority of people since that is the rarest of use cases). 

Again, the affected group that this fixes is for people who have 'test' in the server field and  have test mode OFF, and want to submit transactions that return transaction_ids during testing. Live production should be unaffected by the change.