'Complete the process of switching to encrypted communication.
This callback function runs after the response to the STLS command has
been received.
The final steps are discarding the cached capabilities and initiating
TLS negotiation on the transport.
@type result: L{dict} mapping L{bytes} to L{list} of L{bytes} and/or
L{bytes} to L{None}
@param result: The server capabilities.
@type context: L{ClientContextFactory
<twisted.internet.ssl.ClientContextFactory>}
@param context: A context factory with which to negotiate TLS.
@type tls: L{ITLSTransport <interfaces.ITLSTransport>}
@param tls: A TCP transport that supports switching to TLS midstream.
@rtype: L{dict} mapping L{bytes} to L{list} of L{bytes} and/or L{bytes}
to L{None}
@return: The server capabilities.'

def _startedTLS(self, result, context, tls):

    self.transport = tls
    self.transport.startTLS(context)
    self._capCache = None
    self.startedTLS = True
    return result
