[whatwg/fetch] Safelist request headers starting with `Sec-` (#880)

As discussed with @annevk on https://github.com/w3c/resource-hints/issues/74#issuecomment-473916086, this PR makes sure that `Sec-` prefixed request headers are always CORS-safe.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/880

-- Commit Summary --

  * safelist request headers starting with Sec-

-- File Changes --

    M fetch.bs (8)

-- Patch Links --

https://github.com/whatwg/fetch/pull/880.patch
https://github.com/whatwg/fetch/pull/880.diff

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