use 'buffer' for default JS Kafka encoding

this makes the messages consumed by the JS Kafka Client match the
native librdkafka backend and create buffers for the key and value

this also makes the JS Kafka client match the documentation.

lib/kafka/Kafka.js

@@ -180,7 +180,9 @@ class Kafka extends EventEmitter {
       fetchMaxWaitMs: 100,
       autoCommit: autoCommit,
       autoCommitIntervalMs: 5000,
-      connectRetryOptions: this.connectDirectlyToBroker ? DEFAULT_RETRY_OPTIONS : undefined
+      connectRetryOptions: this.connectDirectlyToBroker ? DEFAULT_RETRY_OPTIONS : undefined,
+      encoding: "buffer",
+      keyEncoding: "buffer"
     };
 
     //overwrite default options