In function Request::fetch_main, flag local_urls_only (if set)
should allow fetching local urls only. Before this change, the flag had
the inverse behaviour.
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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