Re: [fetch] `user-agent` header control (#37)

> userAgent is an option that is undefined, null, or a header value. undefined indicates the network layer inserts a User-Agent header with a default value. null means the network layer does nothing. A header value means the network layer inserts User-Agent header with that value.

+1 to this route. I agree with @domenic's comment on "mergeMultimaps" mental model (https://github.com/whatwg/fetch/issues/37#issuecomment-89957188) as being developer friendly and most intuitive.

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