Unrevert Authorize.net

@ntalbott @odorcicd for review

The [XML schema for Authorize.net](https://api.authorize.net/xml/v1/schema/AnetApiSchema.xsd) needs the `customerIP` element to come after `customer`, `billTo`, and `shipTo` as far as I can tell - I made the change in https://github.com/Shopify/active_merchant/commit/e1bb6fd37cb95e12bb85f5ee9217a8e66e50b725, and then reverted the revert from last night.