[fetch] Rename `response` in HTTP fetch 3.1.1-2 (#54)

https://fetch.spec.whatwg.org/#http-fetch

> 1. Let *response* be the result of performing a CORS preflight fetch using *request*.
> 2. If *response* is a network error, return *response*.

*response* is already used for a variable introduced in step 1, which I think this variable is supposed to shadow. It would be clearer to use "preflight response", say.

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https://github.com/whatwg/fetch/issues/54