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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