Fix properties setting of conv in ctx

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

How should we do that if this attribute is not writable?