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

@annevk So, to quote @sicking here:

> For cross-origin requests we can allow it to be set, but it'd cause a preflight, and would require the server to send a "access-control-allow-headers"

If that means, the request will not even be *sent* with the modified UA in case of cross-origin requests and a failed CORS preflight, then spec-wise this should be fine.

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