[whatwg/fetch] Adjust casing of client hints (#704)

Also consistly make appending these headers a "should".

Fixes #475.


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<a href="https://whatpr.org/fetch/704.html" title="Last updated on Apr 17, 2018, 1:05 PM GMT (3c0d46e)">Preview</a> | <a href="https://whatpr.org/fetch/704/860922f...3c0d46e.html" title="Last updated on Apr 17, 2018, 1:05 PM GMT (3c0d46e)">Diff</a>
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  https://github.com/whatwg/fetch/pull/704

-- Commit Summary --

  * Adjust casing of client hints

-- File Changes --

    M fetch.bs (34)

-- Patch Links --

https://github.com/whatwg/fetch/pull/704.patch
https://github.com/whatwg/fetch/pull/704.diff

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