Fixes https://github.com/spring-cloud/stream-applications/issues/500
When `listeners` are provided for `DefaultKafkaConsumerFactory`,
the target `KafkaConsumer` instance is proxied.
The `java.lang.reflect.Proxy` is `Serializable`,
but the value it is wrapping is not.
When the `MessageHeaders` is serialized (e.g. into persistent `MessageStore`),
it checks for `Serializable` type only on top-level object of the header.
Therefore, the `Proxy` is passing condition, but eventually we fail
with `NotSerializableException`, since the proxied object is not like that
* Remove `kafka_consumer` from a message before it reaches an aggregator
with its logic to serialize message into the store
This is a workaround until Spring for Apache Kafka is released
with the fix: https://github.com/spring-projects/spring-kafka/pull/2822
cf9d8cd79c