[whatwg/fetch] Generic ```Accept``` header value ```*/*``` and Service Workers (#634)

Generic ```Accept``` headers like, for example, ```*/*``` for [images in Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=1249474) make dealing with such requests in a Service Worker context [hard](http://twitter.com/justmarkup/status/926793878950367232) and encourage relying on [file extension sniffing](https://justmarkup.com/log/2017/11/network-based-image-loading/), especially as there is [no consistency](https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation/List_of_default_Accept_values#Values_for_an_image) among how browsers deal with setting this value.

This issue is somewhat related to https://github.com/whatwg/fetch/issues/521 for dynamically created ```Request```s , but also covers static fetches coming from ```img```, ```script```, ```link```, *etc.*

@wanderview [suggested](https://github.com/w3c/ServiceWorker/issues/1206#issuecomment-341991783) to open an Issue with Fetch in order to discuss this.

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