Empty directory in aws key breaks listdir api.

Example: s3://test_bucket//test_path

The double slash '//' is valid part of the key, we were able to validate that for a customer. But due to lstrip in core.py on key, s3 will fail to list without this exact string.

Current behavior

s3.listdir("s3://test_bucket//test_path") -> []

Expected behavior

s3.listdir("s3://test_bucket//test_path") -> [{'Key': 'test_bucket//test_path/dir1', 'Size': 0, 'StorageClass': 'DIRECTORY', 'type': 'directory', 'size': 0, 'name': 'test_bucket//test_path/dir1'}
]

Start in core.py and trace the key handling used by the listdir API for the issue's s3://test_bucket//test_path example. Reproduce the current empty result and verify that listing preserves the double slash and returns the expected directory entry.