Support native serialization in MessageChannelBinder

Resolves #691

- If the broker supports native serialization, then the serialization of the
   outbound payload is not done by the binder's producer binding outbound message handler.
- At the consumer binding's inbound message handler, the extraction of embedded headers
   is performed only for the byte[] payload (given the deserialization would have already
   occurred at the broker level)
- Add correspoinding tests in Kafka binder implementation
- Rely on the received message type to decide on using out of the box deserializer (only
   if byte[] message payload is received)

spring-cloud-stream-core-docs/src/main/asciidoc/spring-cloud-stream-overview.adoc

@@ -1004,7 +1004,13 @@ Effective only for messaging middleware that does not support message headers na
 Useful when producing data for non-Spring Cloud Stream applications.
 +
 Default: `embeddedHeaders`.
-
+useNativeEncoding::
+  When set to `true`, the outbound message is serialized directly by client library, which must be configured correspondingly (e.g. setting an appropriate Kafka producer value serializer).
+When this configuration is being used, the outbound message marshalling is not based on the `contentType` of the binding.
+When native encoding is used, it is the responsibility of the consumer to use appropriate decoder (ex: Kafka consumer value de-serializer) to deserialize the inbound message.
+Also, when native encoding/decoding is used the `headerMode` property is ignored and headers will not be embedded into the message.
++
+Default: `false`.
 [[contenttypemanagement]]
 == Content Type and Transformation