Headers cleanup

Headers type is defined in three places:

• SvelteKit
• Node.js
• node-fetch

We try to pass these around interchangeably, but that’s almost certainly broken since it may be a string | string[] in Node.js, but not in the other two places.

I believe that Node.js has defined headers correctly while the other two are broken. There’s actually an open issue for this in the node-fetch repo https://github.com/node-fetch/node-fetch/issues/771. Unfortunately, I’m not quite sure what to do about this as long as that issue remains open

I think it’s also possible that fetch headers are different than generic request headers? In which case, maybe it’s just SvelteKit that’s broken and the other two are both correct

[Conduitry]

They even used to have a .getAll() that returned an array and decided no that was too helpful.

[benmccann]

Ah, yeah, you’re right about that: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie. So I guess the node-fetch API probably is correct for that use case

I think our own Headers definition is the only one that’s broken then. fetch headers can be string, but I think our Request and Response headers are wrong. E.g. consider the set-cookie example in the documentation - with this API you can only set a single cookie