Fix for issue where SSH daemon does not answer SSH_MSG_CHANNEL_REQUEST for "window-change"

resize_pty(), and Client.invoke_shell().  Perhaps useless, but more RFC
compliant.  Updated methods to include these parameters in server messages.

Adjusted Channel.resize_pty() to neither request nor wait for a response, as
per RFC 4254 6.7 (A response SHOULD NOT be sent to this message.)  This is
necessary as certain hosts have been observed to not acknowledge this type of
channel request (Cisco IOS XR), which causes paramiko to end the session.