'Send an RFC5161 enable string to the server.
(typ, [data]) = <intance>.enable(capability)'

def enable(self, capability):

    if ('ENABLE' not in self.capabilities):
        raise self.error('Server    does    not    support    ENABLE')
    (typ, data) = self._simple_command('ENABLE', capability)
    if ((typ == 'OK') and ('UTF8=ACCEPT' in capability.upper())):
        self._mode_utf8()
    return (typ, data)
