rgw: add COPY method to be handled by CORS

Was missing this http method.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: fix certain return status cases in CORS

Change return values in certain cases, reorder
checks, etc.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>