dj-stripe: PaymentMethod.detach() does not set default_payment_method to None

Describe the bug

I found a bug today when a customer is detaching his PaymentMethod and he only had one.

Detaching works fine because PaymentMethod.customer is set to None, however Customer.default_payment_method still references the now detached PaymentMethod.

To Reproduce

1. Create customer
2. Attach a payment method
3. Detach payment method

Expected behavior

If no other payment methods are available, set default_payment_method back to None. If any other payment method is available, we could pick one of them optionally.

As a workaround I am using the payment_method.detached webhook:

def payment_method_detached(event):
    """ Not sure if it's a bug in djstripe's PaymentMethod.detach() but default_payment_method is not set to None """

    customer_id = event.data["previous_attributes"]["customer"]
    customer = Customer.objects.get(id=customer_id)

    if not customer.payment_methods.all().exists():
        customer.default_payment_method = None
        customer.save()

Environment

• dj-stripe version: 2.3.0
• Your Stripe account’s default API version: 2019-11-05
• Database: Postgres
• Python version: 3.6.9
• Django version: 2.2.11

Can you reproduce the issue with the latest version of master?

[Yes / No]

Additional context Add any other context about the problem here.