~Continue the work done in #14811 and #15547~. And applies the Fetch [spec change](0d9a4db8bc) about [allowing redirects after CORS preflights](https://github.com/whatwg/fetch/issues/204). - [X] `./mach build -d` does not report any errors - [X] `./mach test-tidy` does not report any errors - [X] These changes fix ~#14787 and~ #14519 - [X] There are tests for these changes. Several WPTs go from FAIL to SUCCESS. The new expected FAILs for the cors/redirect-preflight.htm tests are caused by the spec change done [here](0d9a4db8bc), as suggested in the associated [issue](https://github.com/servo/servo/issues/14519#issuecomment-284384667). Source-Repo: https://github.com/servo/servo Source-Revision: c90d3d2f06c03d153b2be3ffa9a941cbfe8508cc