This commit aligns the `fetch` API and the `Request` / `Response`
classes belonging to it to the spec. This commit enables all the
relevant `fetch` WPT tests. Spec compliance is now at around 90%.
Performance is essentially identical now (within 1% of 1.9.0).
This commit fixes the URL returned from `request.url` in the HTTP server
to be fully qualified. This previously existed, but was removed and
accidentially not readded during optimizations of the HTTP ops.
Returning a non fully qualified URL from `Request#url` is not spec
compliant.
