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>
(cherry picked from commit bcf92c496aba0dfde432290fc2df5620a2767313)