Fix listing files support, when using vblob as backend

Fixes a problem when using knox with vblob backend ( https://github.com/cloudfoundry/vblob ).

If vblob is configured to use s3-authentication scheme and user tries to call Client.list()-function of knox, the requests will fail. This is caused by differences in 'resource'-parameter used in Authorization-module. When knox is forming the 'resource' parameter, it uses 'fixedFilename'-variable as one part. If 'filename' is empty 'fixedFilename' will get the value: '/'. This slash is extraneous for 'resource'-parameter, when requesting a list of files and causes vblob to reject the request.

I don't have possibility to test this against _real_ S3 (no credentials), so please ensure that this doesn't break anything by running the tests.