Fix endpoint validation to allow returning string for all content types

[benmccann]

Allows you to do:

export const get = () => {
    const body = '<foo />';
    return { status: 200, headers: { 'content-type': 'application/xml' }, body }
};

[changeset-bot]

🦋 Changeset detected

Latest commit: 945e965

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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