Accept an array of strings or buffers as the header argument for webhooks.constructEvent

[rattrayalex-stripe]

r? @richardm-stripe
cc @stripe/api-libraries

When trying out the new library, Thor found that express's TS types indicate that a header is typed as string | []string.

This resulted in the following type error:

His workaround was to do a cast:

const signature: string = req.headers["stripe-signature"] as string;
event = stripe.webhooks.constructEvent(
  req.body,
  signature,
  process.env.STRIPE_WEBHOOK_SECRET
);

but it'd be nice to not have to do that, and have this Just Work:

event = stripe.webhooks.constructEvent(
  req.body,
  req.headers["stripe-signature"],
  process.env.STRIPE_WEBHOOK_SECRET
);

This turns out to be because express's Request class is a wrapper around http.IncomingMessage, and IncomingMessage#headers claims that:

• set-cookie is always an array. Duplicates are added to the array.
• For all other headers, the values are joined together with ', '.

This means that in practice, this header should never be an array. We shouldn't bother our users with the cast, nor should we handle the array case, which should never occur (at least in http and express, which is what we're adding this for).

So we type it as accepting an array, but throw an error if one is passed.