Make 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": http://awsdocs.s3.amazonaws.com/S3/latest/s3-dg.pdf.

Fixes #1336.