rgw: fix bucket object listing when initial marker matches prefix
When using a delimiter listing on a bucket, when a marker that matches the prefix is provided, no listing will be provided.
Steps to Reproduce:
1. Create a bucket and add an object named "dir1/obj2".
2. Issue a listing of the bucket with a delimiter ("/") with prefix ("dir1/") with marker ("dir/").
Ganesha apparently sets the marker where s3cmd does not, which is why this bug has not yet been discovered.