See http://w3c-test.org/XMLHttpRequest/preserve-ua-header-on-redirect.htm.
<rdar://problem/4741945>
Firefox 56.0 passes <http://w3c-test.org/XMLHttpRequest/preserve-ua-header-on-redirect.htm>. Chrome Canary version 63.0.3231.0 does not pass.
*** This bug has been marked as a duplicate of bug 179527 ***