Add IP address to Optimal gateway requests.

Optimal wants to get IP addresses for fraud verification. There is no difference in the response we get back from them. @bslobodin /cc @Shopify/payments 

@ntalbott I'm sorry about my last two merges being crappy. I'll follow [good practices](http://blog.spreedly.com/2014/06/24/merge-pull-request-considered-harmful) for the next one.