'Do a plaintext login over an encrypted transport.
This callback function runs after the transport switches to encrypted
communication.
@type res: L{dict} mapping L{bytes} to L{list} of L{bytes} and/or
L{bytes} to L{None}
@param res: The server capabilities.
@type username: L{bytes}
@param username: The username with which to log in.
@type password: L{bytes}
@param password: The password with which to log in.
@rtype: L{Deferred <defer.Deferred>} which successfully fires with
L{bytes} or fails with L{ServerErrorResponse}
@return: A deferred which fires when the server accepts the username
and password or fails when the server rejects either.  On a
successful login, it returns the server\'s response minus the
status indicator.'

def _loginTLS(self, res, username, password):

    return self._plaintext(username, password)
