public void SubscriptionPreview_CanCreate()
        {
            // Arrange
            var subscription = Chargify.GetSubscriptionList().FirstOrDefault(s => s.Value.State == SubscriptionState.Active).Value as Subscription;

            Assert.IsNotNull(subscription, "No suitable subscription could be found.");
            var options = new SubscriptionCreateOptions()
            {
                ProductHandle        = subscription.Product.Handle,
                CustomerAttributes   = subscription.Customer.ToCustomerAttributes() as CustomerAttributes,
                CreditCardAttributes = new CreditCardAttributes()
                {
                    FullNumber = "1", ExpirationMonth = 1, ExpirationYear = DateTime.Now.Year + 1
                }
            };

            // Act
            var result = Chargify.CreateSubscriptionPreview(options);

            // Assert
            Assert.IsNotNull(result);
            Assert.IsInstanceOfType(result, typeof(ISubscriptionPreview));
            Assert.AreEqual(1, result.SubscriptionPreviewResult.CurrentBillingManifest.LineItems.Count);
        }