Seeing as how the `InvalidCreditCard` object and the `ValidAccount` object are very close to the same, what do you think about following what some of the other specs in this file are doing by dup'ing the valid object, tweaking one of its fields to invalid, and then validating based off that?

I kind of like this approach because we know that without changes `ValidAccount` will pass validation, so it's nice to see exactly how validation will fail as its properties are tweaked.