Minor log4j fix in the producer

core/src/main/scala/kafka/producer/async/DefaultEventHandler.scala

@@ -239,10 +239,10 @@ class DefaultEventHandler[K,V](config: ProducerConfig,
       try {
         val syncProducer = producerPool.getProducer(brokerId)
         debug("Producer sending messages with correlation id %d for topics %s to broker %d on %s:%d"
-          .format(currentCorrelationId, messagesPerTopic, brokerId, syncProducer.config.host, syncProducer.config.port))
+          .format(currentCorrelationId, messagesPerTopic.keySet.mkString(","), brokerId, syncProducer.config.host, syncProducer.config.port))
         val response = syncProducer.send(producerRequest)
         debug("Producer sent messages with correlation id %d for topics %s to broker %d on %s:%d"
-          .format(currentCorrelationId, messagesPerTopic, brokerId, syncProducer.config.host, syncProducer.config.port))
+          .format(currentCorrelationId, messagesPerTopic.keySet.mkString(","), brokerId, syncProducer.config.host, syncProducer.config.port))
         if (response.status.size != producerRequest.data.size)
           throw new KafkaException("Incomplete response (%s) for producer request (%s)"
             .format(response, producerRequest))