[whatwg/fetch] Document header casing (#972)

As far as I know request header name casing is exposed to servers when using h1 (though you cannot have case-insensitive duplicates with different casing). Order of request headers isn't really defined, though I maybe that means insertion order? 

Response headers are always normalized to lowercase. They are also ordered lexicographically though XMLHttpRequest orders slightly differently (uppercases first).

Some of these characteristics are worth mentioning in a note I think as they are relevant for implementations and it would be good if they didn't have to derive this from the various places that expose headers.

(If h1 disappears this will be less of an issue since presumably future iterations of the protocol won't reintroduce design issues like this.)

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