rgw: Look at correct header about headers for CORS

rgw: Look at correct header about headers for CORS

The CORS standard dictates that preflight requests are made with the
Access-Control-Request-Headers header containing the headers of the
author request. The Access-Control-Allow-Headers header is sent in the
response.

The present code looks for Access-Control-Allow-Headers in request, so
fix it to look at Access-Control-Request-Headers instead.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 2abacd9678ae04cefac457882ba718a454948915)