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 aligns `Headers` to spec. It also removes the now unused
03_dom_iterable.js file. We now pass all relevant `Headers` WPT. We do
not implement any sort of header filtering, as we are a server side
runtime.
This is likely not the most efficient implementation of `Headers` yet.
It is however spec compliant. Once all the APIs in the `HTTP` hot loop
are correct we can start optimizing them. It is likely that this commit
reduces bench throughput temporarily.
