[fix] adapter-cloudflare-workers: support setting multiple values to a header

[JeanJPNM]

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

Closes #2305 by using Headers.append on headers that have multiple values.

[changeset-bot]

🦋 Changeset detected

Latest commit: 2463529

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-cloudflare-workers	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[half2me]

@JeanJPNM do you think it would be worth to check if headers is an instance of Headers and if so, just leave it as it is. Currently this breaks if headers is already a Headers object.

I only noticed this because I had a workaround in my app which returned a Headers object, and that doesn't work with the latest version now.

[JeanJPNM]

That's supposed to be the original behavior, headers was supposed to be a plain object. being able to return an instance of Headers was a side effect of the implementation of adapter-cloudflare-workers.

[half2me]

@JeanJPNM okay, makes sense :)