Allow for redirects after a CORS-preflight
~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).
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