SftpClient.IsConnect returns true even if session is closed.

[anandgmenon]

This is similar to the discussion in #843

We have a connection pool implemented for the SftpClient and we reconnect whenever SftpClient.IsConnected() returns false.
However we have noticed issues where SftpClient.IsConnected() still returns true in case the server has closed the session and we eventually get the following error when using the client for any operation

System.InvalidOperationException: The session is not open. at Renci.SshNet.Common.AsyncResult.EndInvoke() at Renci.SshNet.SftpClient.EndListDirectory(IAsyncResult asyncResult)

Is this an expected behaviour? If not, can we try to implement the fix mentioned #843 or expose a method to determine if the session is open or not?