Fix StripeResource.extend type

[rattrayalex]

I have access to a beta feature, which I wanted to use with the stripe library.

In the process, I found that the result of Stripe.StripeResource.extend() did not have a constructor, so TypeScript would not let you do this:

const Foo = Stripe.StripeResource.extend(); 
new Foo(stripe)

This fixes that.

For context, I use this like so:

class Foos extends Stripe.StripeResource.extend({ path: "foos" }) {
  create = Stripe.StripeResource.method({ method: "POST", path: "" }) as (
    params: Stripe.FooBetaFeatureCreateParams,
    options: Stripe.RequestOptions
  ) => Promise<Stripe.Response<Stripe.FooBetaFeature>>;

  update = Stripe.StripeResource.method({ method: "POST", path: "/{foo}" }) as (
    id: string,
    params: Stripe.FooBetaFeatureUpdateParams,
    options: Stripe.RequestOptions
  ) => Promise<Stripe.Response<Stripe.FooBetaFeature>>;

  retrieve = Stripe.StripeResource.method({ method: "GET", path: "/{foo}" }) as (
    id: string,
    options: Stripe.RequestOptions
  ) => Promise<Stripe.Response<Stripe.FooBetaFeature>>;
}

class StripeWithFooBeta extends Stripe {
  foos: Foos;

  constructor(key: string, opts: Stripe.StripeConfig) {
    super(key, opts);
    this.foos = new Foos(this);
  }
}

export const stripe = new StripeWithFooBeta(process.env.STRIPE_SECRET!, {
  apiVersion,
});

declare module "stripe" {
  namespace Stripe {
    interface FooBetaFeature {...}
    interface FooBetaFeatureCreateParams {...}
    interface FooBetaFeatureUpdateParams {...}
  }
}

since the transformation of this.path which takes place here does not play nicely with ES6 classes.

[CLAassistant]

All committers have signed the CLA.

[richardm-stripe]

Hello Alex! Thanks for the contribution and the detailed description. This looks good -- I added an entry in typescriptTest.ts and am merging. This will be bundled with the next release.