fix(5383): undici works better with two arguments

[GauBen]

Fixes #5383

I don't know why this works better, but something is wrong on undici's side at step 25:

https://github.com/nodejs/undici/blob/5ca25c23500d649a629865d9a7a28ce653d5e991/lib/fetch/request.js#L321-L341

Specifically, when only one argument is given (as before this PR), init.method gets a default value of GET and erases request.method.

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After a bit of research, this was fixed in nodejs/undici#1529

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For people that want a quick workaround: patch node_modules/@sveltejs/kit/assets/server/index.js:2304 with:

const external_request = new Request(requested, /** @type {RequestInit} */ (opts));
response = await options.hooks.externalFetch.call(null, external_request.url, external_request);

and restart the dev server.

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I'll open the PR nonetheless because svelte-kit seems to release faster than undici.

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[GauBen]

I thought this would be a quick fix, but I don't know the implications of changing this type:

https://github.com/GauBen/kit/blob/41a5a2cb760e70a10bc7237542f95363678564f0/packages/kit/types/index.d.ts#L157-L159

Probably a big deal because it's in the public-facing API. (Edit: indeed)