Created Issue: IsConnected returns true when sshd has closed connection or network is not available [1898]

Currently, The IsConnection property (on BaseClient) returns true even when:
* the server has shut down the ssh connection

To reproduce:

1. Create a connection (eg using SftpClient)
2. Kill sshd process on server.
3. Inspect value of IsConnected property.

* the network is no longer available

To reproduce:

1. Create a connection (eg using SftpClient)
2. Unplug network cable
3. Inspect value of IsConnected property.

I can reproduce this with ssh.net built from SVN/TFS.