'Registering a streaming producer twice throws an exception.'

def test_streamingProducerTwice(self):

    (clientProtocol, tlsProtocol, producer) = self.setupStreamingProducer()
    originalProducer = tlsProtocol._producer
    producer2 = object()
    self.assertRaises(RuntimeError, clientProtocol.transport.registerProducer, producer2, True)
    self.assertIs(tlsProtocol._producer, originalProducer)
