Re: [fetch] CSP Request Header and CORS preflight fetch. (#52)

There's a similar known issue regarding the EventSource (https://html.spec.whatwg.org/multipage/comms.html#last-event-id). It sends out Last-Event-ID. It's currently treated as a non-simple header and Chromium's ServiceWorker would issue a preflight for it  (cc/ @horo-t).

In a different context, we're about to exempt the `Save-Data` header (http://httpwg.org/http-extensions/client-hints.html#the-save-data-hint, https://code.google.com/p/chromium/issues/detail?id=584889). It's currently experimental and specific to Chromium but planned to be standardized.

There could be more stuffs like this coming that are intended to be observable/alterable by a ServiceWorker.

For the Last-Event-ID, I think we can just add it to the simple header list and leave it open to XHR/Fetch (i.e. leave it not forbidden).

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