Allow Responses to be constructed with a body/headers

[jifalops]

The platform Response type has a default constructor.

I am trying to write an interface that can cache to a File or browser cache and am a bit stuck without the ability to construct a response myself. I have a workaround in mind but it breaks the semantics of the cache being a local resource since I'll have to allow it to handle the fetching so it can instantiate a Response.