Fixing an issue with deleting nested S3 folders
Fixing an issue with deleting nested S3 folders

When deleting a nested folder, we first list the objects in the folder
using a key prefix. The key prefix was actually returning the place
holder key created in the AWS Management Console (e.g., 'foo/'), which
failed the test that ensures the directory is empty. We are now delting
the placeholder key and ensuring it is the only key in the folder.

Closes #328.

Avantians/aws-sdk-php,kmidev38/aws-sdk-php,cjyclaire/aws-sdk-php,BoXHocK/aws-sdk-php,piotrpasich/aws-sdk-php,Briareos/aws-sdk-php,oswaldderiemaecker/aws-sdk-php,ilons/aws-sdk-php,Briareos/aws-sdk-php,easybiblabs/aws-sdk-php,jeskew/aws-sdk-php,MaxKlyukin/aws-sdk-php,andythorne/aws-sdk-php,aws/aws-sdk-php,femans/aws-sdk-php,jeskew/aws-sdk-php