rgw: fix url escaping

Fixes: #8202
This fixes the radosgw side of issue #8202. Needed to cast value
to unsigned char, otherwise it'd get padded.

Backport: dumpling

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1734 from ceph/wip-8202

rgw: fix url escaping

Reviewed-by: Sage Weil <sage@inktank.com>