rgw: escape prefix correctly when listing objects

Fixes: #5362
When listing objects prefix needs to be escaped correctly (the
same as with the marker). Otherwise listing objects with prefix
that starts with underscore doesn't work.
Backport: bobtail, cuttlefish

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>