Makes AWS-S3 use the right directory suffix

This change makes AWS-S3 use the right directory suffix. Without this change, calling createDirectory on AWS-S3 with say directory name "someDir" causes it to create a blob named "someDir_$folder_". 

This is different from what happens when one goes through the "Create Folder" workflow from the management console. The AWS-S3 documentation says that when a user creates a folder from the management console, , the console creates an object with the key "/". Note the trailing '/' delimiter. See "Basics of Buckets and Folders" at http://awsdocs.s3.amazonaws.com/S3/latest/s3-dg.pdf.

Fixes #1336.