[fetch] Header value validation (#100)

Per @youennf in https://github.com/w3c/web-platform-tests/pull/2045 WebKit changed how they validate header values based on a new set of HTTP RFCs that are backwards incompatible. Somewhat unexpected from the IETF, but I guess it's a "bigger than browsers" thing.

So now we need to decide what to do with whitespace in various APIs. It seems the sanest solution is to just strip whitespace, especially for `XMLHttpRequest`, but also for the `Headers` object, since there's no semantic difference anyway.

Unless someone proposes a different design I will get this changed for both specifications.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/100