fix: handle empty Headers when serialising Request passed to fetch

[eltigerchino]

fixes #10474

This PR aligning the difference between the server and the client code when reconstructing the Request object passed to the fetch function. Currently, the server returns undefined if there are no headers in the request before data serialisation...

kit/packages/kit/src/runtime/server/page/load_data.js

Lines 227 to 230 in 143dbf9

	const cloned_headers =
	input instanceof Request && [...input.headers].length
	? new Headers(input.headers)
	: init?.headers;

however, the client code simply uses an empty Headers object when building the selector used during hydration.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13023-svelte.vercel.app/

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