INT-3480: public DefaultPahoMC.setPayloadAsBytes

JIRA: https://jira.spring.io/browse/INT-3480

In docs, it s said:
```
The DefaultPahoMessageConverter can be configured to return the raw byte[] in the payload by declaring it as a <bean/> and setting the payloadAsBytes property.
```
http://docs.spring.io/spring-integration/docs/4.0.0.M3/reference/html/mqtt.html