Re: [whatwg/fetch] Some servers seem to expect 'Accept-Encoding : identity' to serve Range requests (#747)

Ok, I have done research! https://github.com/jakearchibald/accept-encoding-range-test

I took all the URLs from HTTP Archive's latest crawl that end in `.mp3` or `.mp4` and tested them with various `Accept-Encoding`s.

**Accuracy warning:** A lot of these URLs seem to return different results each time. I think it can depend on whether the resource is cached at the CDN or not.

Repo: https://github.com/jakearchibald/accept-encoding-range-test.

Results: https://jakearchibald.github.io/accept-encoding-range-test/ (Chrome only due to transform streams).

3.21% of requests fail to return a 206 specifically if there's a non-identity encoding specified.

0.36% of requests fail to return a 206 specifically if the identity encoding is specified, or no encoding is provided. A few of these look like one-time errors, but some are reproducible.

Based on that, it seems like Safari, Firefox, & Chrome are doing something right. If we spec any of them, it should be `Accept-Encoding: identity`. It's the shortest, and most specific.

The repo includes a command to rerun the test for a particular URL if you want.

@annevk Should this kind of weirdness be written into the spec? 3% is a lot.

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