implements data-url fetching
Fixes#10165
jdm mentioned of decoupling the, payload loading mechanism to data_loader.rs. So accordingly a `decoder` method has been added to data_loader.
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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).
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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