[whatwg/fetch] Use "append" instead of "set" to modify the header list (#758)

Per @youennf at https://github.com/whatwg/fetch/issues/747#issuecomment-395148588 Fetch uses "set" in three places (CORS-preflight fetch, `Response.redirect()`) where it could use "append" instead.

It seems worth using "append" consistently.

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