rgw: Fix S3 auth when using response-* query string params

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
Signed-off-by: Sylvain Munaut <s.munaut@whatever-company.com>
(cherry picked from commit 7a7361d7e798d94796d4985e2e4b35af22463ae2)
Merge pull request #808 from ceph/wip-6152-dumpling

rgw: Fix S3 auth when using response-* query string params