When fetching about:blank, response body should be the empty byte
sequence.
Spec: https://fetch.spec.whatwg.org/#concept-basic-fetch
Before this change, response body would be set to `ResponseBody::Empty`,
and then fetching would result in an infinite loop at step 19 in fn
`main_fetch` (methods.rs).
Add Tests For Asynchronous Fetch Calls With Filtered Responses
I've added two tests using async_fetch intended to create filtered responses. I caught and fixed a bug where waiting for response.body to be completed would never pass for Opaque or OpaqueRedirect, since their response.body is always `Empty`.
As always, I'd appreciate review and feedback! I don't plan to write new tests, but I will gladly patch up any gaps in the current tests.
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