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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Test filtered responses and implement Cors Check Fetch step
I've been writing tests for creating filtered responses. So far I have three of the four types being made (namely, Basic, CORS, and Opaque), and just need to figure out how to make an OpaqueRedirect filtered response, since it's handled separately from the others. I will also add more tests to ensure the content of the filtered responses matches the limitations placed by the specification.
Along the way I implemented Cors Check, since it's required for the CORS filtered response. @jdm suggested I handle it in here, since it's such a small step, compared to other parts of Fetch.
Since all the tests currently pass, and I've spent a while adding the Cors Check and other pieces, I figured now would be a good time to start having it reviewed.
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