Raise immediate error on producer.send after close (#2542)

Author: dpkp

kafka/producer/kafka.py

@@ -594,6 +594,7 @@ def send(self, topic, value=None, key=None, headers=None, partition=None, timest
             KafkaTimeoutError: if unable to fetch topic metadata, or unable
                 to obtain memory buffer prior to configured max_block_ms
         """
+        assert not self._closed, 'KafkaProducer already closed!'
         assert value is not None or self.config['api_version'] >= (0, 8, 1), (
             'Null messages require kafka >= 0.8.1')
         assert not (value is None and key is None), 'Need at least one: key or value'