Fixes a pesky bug in the fetch implementation where if the init part is
specified in `fetch` instead of the `Request` constructor, the
fillHeaders function receives two references to the same object, causing
it to append to the same list being iterated over.
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).
