Bug 211.

When performing upload with chunking into 4M blocks BlobWriteStream::WriteBlockAsync calls PutBlockAsync without async declaration. This caused one of the final blocks (second last usually) to not be uploaded correctly. This in turn caused the noPendingWritesEvent  ManualResetEvent to always have the value of 1 (since the write wasn't complete). This would "hang" at the WaitOne method called and blob would never be reconstructed properly.

Have added "async" call to PutBlockAsync, all blocks write correctly and no hanging occurs.