Download with deleteRemoved option should not delete subdirectory

Fixes #54

To sum it up, when you try to download a remote directory called `remote-dir` from a bucket, and specify the local directory as `local-dir`, I would expect just the contents of `remote-dir` to download, not the directory itself, so `remote-dir/` shouldn't need to be pushed to `allS3Objects` when listing remote objects, but it is right now.

If `local-dir` has a subdirectory called `foo` (valid directory that also exists remotely and thus should be retained), it will get deleted on the next download attempt. Because of the extra `remote-dir/` entry in `allS3Objects`, when you start diff'ing the remote objects against the local ones, you end up checking if the string `foo/` is contained within the `remote-dir/` s3Object.key string, which is an empty string after the [prefix truncation](https://github.com/andrewrk/node-s3-client/blob/4.3.1/lib/index.js#L1154). And since it isn't, [`deleteLocalDir()`](https://github.com/andrewrk/node-s3-client/blob/4.3.1/lib/index.js#L953) gets called.