rgw: Fix CORS allow-headers validation

rgw: Fix CORS allow-headers validation

This fix is needed because Ceph presently validates CORS headers in a
case-sensitive manner. Keeps a local cache of lowercased allowed headers
to avoid converting the allowed headers to lowercase each time.

CORS 6.2.6: If any of the header field-names is not a ASCII
case-insensitive match for any of the values in list of headers do not
set any additional headers and terminate this set of steps.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>