docs: handle immutable headers error

[hyunbinseo]

Fixes #11883. Stumbled on it again after 5 months.

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[Conduitry]

I don't think this is the proper way to suggest handling this. If it's important that this header be set, you don't want to just be silently failing and ignoring that. See e.g. the way this is handled in #10030.

[hyunbinseo]

It definitely is not a proper way to handle this problem.

However, the current documentation is prone to runtime errors.

And I didn't want to mention cloning responses just for this exception.

// the returned Response might have immutable Headers
// so we should clone them before trying to mutate them
response = new Response(response.body, {
  status: response.status,
  statusText: response.statusText,
  headers: new Headers(response.headers)
});

@Conduitry Any suggestions on how to improve the documentation?

Maybe provide an example other than response.headers.set()?

After-all, it is just a server handle function example code.

[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-12543-svelte.vercel.app/

this is an automated message

[eltigerchino]

Thank you!