'L{TransportProxyProducer._stopProxying} drops the reference to the
wrapped L{IPushProducer} provider.'

def test_stopProxyingUnreferencesProducer(self):

    transport = StringTransport()
    proxy = TransportProxyProducer(transport)
    self.assertIdentical(proxy._producer, transport)
    proxy._stopProxying()
    self.assertIdentical(proxy._producer, None)
