async updateSubscriptionDefaultPaymentMethod(subscription, paymentMethod) {
        await this._rateLimitBucket.throttle();
        return await this._stripe.subscriptions.update(subscription, {
            default_payment_method: paymentMethod
        });
    }
Update the default PaymentMethod for a Subscription.

@param {string} subscription
@param {string} paymentMethod

@returns {Promise<ISubscription>}