The custom partitioner callbacks are not invoked

[didier-wenzek]

The partitioner callback set when a topic producer is created is not used when messages are produced with no partition assigned.

Setting a partitioner callback as in:

let partitioner_callback partition_cnt key = Some ((Hashtbl.hash key) mod partition_cnt) in
let topic = Kafka.new_topic ~partitioner_callback producer topic_name options in                                                                                                                                 
Kafka.produce topic ~key msg

we expect that the partitioner_callback is called for each message produced without a partition.
Actually, the custom callback is never invoked and the partition is assigned using the partitioner set by the partitioner property