[fetch] Improve HSTS language (#244)

When I added HSTS support to the WebSocket API I used better HSTS language than is in use in Fetch. However, I never backported that.

See https://github.com/whatwg/html/pull/840 for that language (where it'll be removed from HTML).

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/244