* removed hard-coded Kafka key and value deserializer, leaving default deserializer as org.apache.kafka.common.serialization.ByteArrayDeserializer. Also added checks to ensure that any provided deserializer class extends org.apache.kafka.serialization.Deserializer and outputs a byte array.
* Addressed all comments from original pull request and also added a
unit test.
* Added additional test that uses "poll" to ensure that custom deserializer
works properly.