Header value validation

[annevk]

Per @youennf in web-platform-tests/wpt#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.