XD-1016: Provide an option to pretty print JSON output

JIRA: https://jira.spring.io/browse/XD-1016
* add pretty print to JSON output
* add test case
are you suggesting provide attribute prettyPrint in <bean class="org.springframework.xd.dirt.integration.bus.converter.PojoToJsonMessageConverter" prettyPrint="true"/>?
Where should we use property "typeconversion.json.pretty_print" then?