[whatwg/fetch] Image Accept: headers in the spec prevent conneg (#877)

The fetch spec pretty much mandates that everyone uses:
`image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5` for image requests.
This should be adapted to give user agents the possibility to include image formats that might not be universally accepted but preferred, e.g. `image/webp` or `image/apng` or `image/jxr` to allow [conneg](https://en.wikipedia.org/wiki/Content_negotiation) to happen with servers that support multiple other formats.

As it stands, the spec doesn't allow for this kind of required flexibility, and users agents would not be spec compliant if they indicate their capabilities to servers.



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