Refactor the Response class so that we can support internal responses.

[serverhiccups]

Spec: https://fetch.spec.whatwg.org/
Currently, we support the idea of a filtered response by just omitting information information from the response. In order to properly meet the spec, we must have an internal response. This should also solve an issue where "cors" type requests don't use Access-Control-Expose-Headers.

I think that this is pretty low priority as the current implementation will be fine for 99% of use cases.

[stale]

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