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.
This commit renames two assertion functions to better align with JS API:
- assertStringContains -> assertStringIncludes
- assertArrayContains -> assertArrayIncludes
