Re: [fetch] Update headers upon a 304; fixes #97 (#141)

I feel like 

> If request's cache mode is "default" and request's header list contains a header named `If-Modified-Since`, `If-None-Match`, `If-Unmodified-Since`, `If-Match`, or `If-Range`, set request's cache mode to "no-store".

should go in there too. Any objection?

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https://github.com/whatwg/fetch/pull/141#issuecomment-149463328