Subscribing a user to a free plan without supplying cc details

Although Stripe will allow you to create a free plan (with an amount 0) and then subscribe a customer to that plan without supplying cc details, it doesn't appear possible with dj-stripe. 

`SubscribeFormView.post()` requires that a "stripe_token" is supplied to update the customers card, if not an error is thrown.

```
You cannot set card to an empty string. We interpret empty strings as None in requests.You may set
```

Is there a specific reason for this? Do other parts of dj-stripe require that a customer has a cc associated with them?