BillingInfo expects a reference to the account

We got a support request where `buildTransactionForm` was building a form that wouldn't save. It would get to [`BillingInfo.toJSON`](https://github.com/recurly/recurly-js/blob/7ce06a37e25e588444e377418d576de239de3043/build/recurly.js#L752) and die because it didn't have a `firstName` and  `this.account` was `null`.