INT-3295 Datatype Channel : Use MessageConverter

JIRA: https://jira.springsource.org/browse/INT-3295

Instead of invoking the conversion service directly, do it
via a MessageConverter (DefaultDatatypeChannelMessageConverter).

That way, users can override the MessageConverter; for example,
XD wants to look at the content-type header during the conversion
process (and update the content-type).