Add prorate option to subscription creation options

with tests

types/stripe/index.d.ts

@@ -4470,6 +4470,12 @@ declare namespace Stripe {
              * with month or year intervals, the day of the month for subsequent invoices.
              */
             billing_cycle_anchor?: number;
+
+            /**
+             * Boolean (default true). Use with a billing_cycle_anchor timestamp to determine whether the customer will be invoiced a prorated amount until
+             * the anchor date. If false, the anchor period will be free (similar to a trial).
+             */
+            prorate?: boolean;
         }
 
         interface ISubscriptionCreationOptions extends ISubscriptionCustCreationOptions {

types/stripe/stripe-tests.ts

@@ -252,7 +252,7 @@ stripe.customers.create({
     customer.cards.del("card_17xMvXBoqMA9o2xkq6W5gamx").then(function (confirmation) {});
 
     customer.subscriptions.create({ items: [{ plan: "gold" }], trial_period_days: 7 }).then(function (subscription) { });
-    customer.subscriptions.create({ items: [{ plan: "gold" }], trial_end: "now", billing_cycle_anchor: 1516881177 }).then(function (subscription) { });
+    customer.subscriptions.create({ items: [{ plan: "gold" }], trial_end: "now", billing_cycle_anchor: 1516881177, prorate: true }).then(function (subscription) { });
     customer.subscriptions.create({ items: [{ plan: "gold" }], trial_end: 1516881177, billing: "send_invoice", days_until_due: 7 }).then(function (subscription) { });
     customer.subscriptions.create({ items: [{ plan: "gold" }], billing: "charge_automatically" }).then(function (subscription) { });
     customer.subscriptions.retrieve("sub_8Eluur5KoIKxuy").then(function (subscription) {