Fix incorrect method name from !1093.

Closes #678 (closed).

There was a thinko in !1093 (merged) where I accidentally called gwcelery.kafka.KafkaListener.get_payload_output instead of gwcelery.kafka.KafkaListener.get_payload_content. This function, set here, unpacks the contents of the Kafka message. This is needed because the avro and JSON serialized messages are packed slightly differently.