Fix Cloudflare Workers adapter issues

[leesus]

Prevents the Cloudflare Workers adapter re-stringifying a JSON request body after parsing with await request.json().

Currently, defensive code needs to be written when running locally vs running in a worker, e.g.

export function post(request) {
  let data;

  try {
    // Workers
    ({ data } = JSON.parse(request.body));
  } catch (e) {
    // Locally
    ({ data } = request.body);
  }
  
  ...
}

instead of

export function post(request) {
  const { data } = request.body;

  ...
}

Converts Headers object to POJO prior to passing to render method, so that getContext hooks etc can access properties via dot notation, rather than using get/has Headers methods.

[benmccann]

@leesus can you add a changeset? (i.e. run pnpx changeset)

[leesus]

@benmccann done 🙂

[halfnelson]

Looks good, as long as the headers object still matches the type Def's from kit

[leesus]

@halfnelson this converts the Headers instance from Workers into a plain JS object, which looks to be correct as per types.internal.d.ts (Record<string, string>).