S3 DownloadDirectory infinite block - Released the Semaphore when key ends with "/"

Problem:
The `TransferUtility.DownloadDirectory()` will hang if you have too many keys in S3 that ends with "/".

Solution:
It seems the Semaphore is being filled and never released when the S3 Key ends with "/". I have added a Release() to fix the issue.

I am no expert on Semaphore, hopefully this doesn't break anything else. 
If there's a better way to prevent infinite wait on the  `await asyncThrottler.WaitAsync(cancellationToken).ConfigureAwait(continueOnCapturedContext: false);` in this particular scenario, let me know and I'll glady send another pull request.