Exception when reading sftp extensions #726

Hello,

I encounter the following exception reading the SFTP extensions => An item with the same key has already been added

The server is returning a duplicate extension : posix-rename@openssh.com which make failed the dictionary Add in ReadExtensionPair().

I know the server shouldn't return twice the extension but I think it would be good to not fail and maybe to keep the last key/value pair received.

I can submit a fix. What do you think ?

Regards

---------- EXCEPTION -------------
System.ArgumentException: An item with the same key has already been added.
at Renci.SshNet.SubsystemSession.WaitOnHandle(WaitHandle waitHandle, Int32 millisecondsTimeout)
at Renci.SshNet.Sftp.SftpSession.OnChannelOpen()
at Renci.SshNet.SubsystemSession.Connect()
at Renci.SshNet.SftpClient.OnConnected()
at Renci.SshNet.BaseClient.Connect()