Previously, opaque responses where handled a little differently than
other responses from the mock server. More specifically, they were not
tracked (so no assertions could be made for them) and their
[`Body` mixin][1] methods (such as `arrayBuffer()`, `json()`, `text()`)
would throw an error due to `body` being `null`.
This commit ensures opaque responses are also captured on the mock
server and also changes `Body` mixin methods to better simulate the
[spec'd behavior][2].
(These improvements will be necessary to test caching of opaque
responses in a subsequent commit.)
[1]: https://developer.mozilla.org/en-US/docs/Web/API/Body
[2]: https://fetch.spec.whatwg.org/#concept-body-consume-body
PR Close#30977
5306330d85