Test the data being sent in auth.net calls in more detail

@jnormore @ntalbott @girasquid this is a WIP, but having assertions like these would have caught the recent auth.net breakage.

There are no tests for the full request to auth.net still so this is just a start, it would be great if someone could expand on this a bit more. We should also audit the other gateways and make sure other gateways have assertions on the data being submitted. It might be good to add an example in the generators as well.