Allow appending multiple Set-Cookie Headers

[nnelgxorz]

Is your feature request related to a problem?

The current headers implementation uses an append function that concatenates header values together with a ", ". This works for all headers except Set-Cookie.

Describe the solution you'd like

There are a few options I can think of:

• create special functions for each headers method specifically for cookies: getCookie, setCookie, appendCookie.
• keep the existing headers methods, but check for the header value internally and store cookies separately. (if (name === 'Set-Cookie) {...}`
• keep everything the same internally, but special case Set-Cookie to values on the ", " before creating the response and generating valid Set-Cookie headers for each value.
• same as above, but use node-fetch's .raw() to get the Set-Cookie headers as an array.

Describe alternatives you've considered

Additional context

whatwg/fetch#973 - Discussion around formalizing a solution for Set-Headers
https://github.com/withastro/astro/pull/3092/files - Astro PR using the adding multiple Set-Cookies for Netlify
https://github.com/sveltejs/kit/pull/3502/files - Svelte PR adding multiple Set-Cookies

[manucorporat]

I think good cookie support is super important, wanna take into preparing a PR most likely inspired by sveltekit and astro?

This would be such a massive improvement!!

[nnelgxorz]

Definitely! I was going to take it on yesterday, but it looks like Qwik City e2e tests are broken. I'm guessing because of the refactor Adam did on the middlewares/servers.

[nnelgxorz]

Opened a PR at #1864

On paper, I think this would fix the issue, but without being able to run tests, I don't feel comfortable merging yet.

[nnelgxorz]

PR is ready for review!