NewAKString is effectively the default for any new IDL interface, so
let's mark this as the default behavior. It also makes it much easier to
figure out whatever interfaces are still left to port over to new AK
String.
Since BodyInit and Headers are tightly coupled to both Request and
Response, I chose to do all of them at once instead of introducing a
bunch of temporary conversion glue code.
This is a normative change in the Fetch spec.
See: e4d3480
This also implements the changes to the 'sort and combine' algorithm,
which now treats "set-cookie" headers differently, and is exposed to JS
via the Headers' iterator.
Passes all 21 WPT tests :^)
http://wpt.live/fetch/api/headers/header-setcookie.any.html
