UserAgent: Mozilla/5.0 (X11; Linux x86_64; rv:48.0) Gecko/20100101 Firefox/48.0

Example URL:

Steps to reproduce the problem:

 - Run `nc -l 9001` locally to capture the redirect
 - Open http://manishearth.anapnea.net/tmp/test-redirect.html
 - Note that a preflight doesn't happen, but the nonstandard x-pony content-type is scrubbed

What is the expected behavior?
A preflight fetch should occur on localhost:9001, to check if the content-type header is an allowed one

See https://fetch.spec.whatwg.org/#cors-safelisted-request-header and https://fetch.spec.whatwg.org/#concept-http-fetch step 4

What went wrong?
Instead of a preflight fetch, it did a direct GET request with the content-type header scrubbed.

Did this work before? N/A 

Chrome version:   Channel: n/a
OS Version: 
Flash Version: Shockwave Flash 11.2 r202

Firefox does this wrong too (differently), corresponding bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1278493