Fix bug where we try to use the card data before it exists; fix specs so they pass

Here's what was going on:

When we call Stripe::Token.create, stripe-mock can take one of two paths, depending on whether we gave it a customer or a card.  If we give it a customer, it looks up that customer's default card.

We were originally giving it a customer, but that customer had no default card, so it was breaking.  In the course of the BSD integration work, we switched to giving it a card, which made stripe-mock happy, but broke in production because we don't have a card at that point.  So now I've switched us back to giving it a customer, and set the mocked customer to have a default card.  So everything should work now :D
Should probably set the card to nil here, so that the specs will catch errors like the one we fixed here.