avoid re-instantiating Response when not required

[sassanh]

The problem is described here: #894
You may ignore the rest and just read the last comment

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[sassanh]

generateResponse uses this function, but it already does what createMutableResponse does when needed.

It is also used here packages/next-on-pages/templates/_worker.js/routes-matcher.ts:239, but since my app is not using middlewares I couldn't test it, npm run test:unit passes though.
If it is needed there, maybe we can do createMutableResponse locally for it?

Hopefully my changes are not fundamentally against your design, I just went with common sense based on my very limited knowledge on the codebase.

[sassanh]

I changed it to

	if (resp.status === 101) {
		return resp;
	}
	return createMutableResponse(resp);

again this is an attempt to allow websocket connections without knowing much about the codebase, I don't know if it is only status code 101 we should put this exception for, or if it is the correct signal for the exception.