Improve RdKafka\Conf::dump

[nick-zh]

When dumping RdKafka\Conf you only see the global settings.
Every rd_kafka_conf_t can also have a rd_kafka_topic_conf_t,
so it might be confusing to not find certain settings in the dump,
that were set through RdKafka\Conf::set like auto.offset.reset.
The behaviour should probably be improved, so that if a rd_kafka_topic_conf_t exists, it should be dumped as well.