[fix] Use headers.raw() on split_headers

[JeanJPNM]

Please don't delete this checklist! 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

Fixes #4087 by using the headers.raw() method from node-fetch to get the array of cookie headers.

[changeset-bot]

🦋 Changeset detected

Latest commit: 8832e00

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-netlify	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

[Auroratide]

Interesting! Didn't know about the raw() method, as it's specific to node-fetch.

I had started on a branch of my own by adding some testing, as there didn't seem to be any for the adapter. Are tests like these desired for a function of this scope?

[Rich-Harris]

thanks! definitely not opposed to adding some tests — our convention for unit tests is to have e.g. a headers.spec.js file alongside headers.js rather than putting them all in a central test.js file.