fetch() should fulfill on success headers, not complete body

[jakearchibald]

See w3c/ServiceWorker#112, but the method to get the full body is pretty undefined, but I'll leave this ticket open as a reminder for when it gets resolved in the spec.

The reason for this is streaming. respondTo(fetch(url)) should be able to stream the response to the browser rather than waiting for the whole response before it can fulfill.

This means we need another, nice-and-easy way to get the response. Hopefully the streams API provides this.