fix: fetch doesn't respect headers from 1st arg if its type is Request

[codercms]

Fixes compatibility with libraries that call the fetch API as fetch(new Request(...someParams)) without an init argument

One of these libraries is https://github.com/sindresorhus/ky (https://github.com/sindresorhus/ky/blob/main/source/utils/timeout.ts)

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[changeset-bot]

🦋 Changeset detected

Latest commit: 7fefd9a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[ghostdevv]

Should we have a changeset here?

[codercms]

@ghostdevv done :)