INT-3369: add NoOpMessageConverter

JIRA: https://jira.spring.io/browse/INT-3369
* add NoOpMessageConverter
* add test
First of all I don't see difference between this and existing `DefaultMessageConverter`.
Another point this converter doesn't do what it implies by this JavaDoc.
From other side, if we talk about `NoOp` for the `SyslogReceivingChannelAdapterSupport` I see it would be better to allow the `null` for the `converter` and it really returns a `message` as is from the underlying `onMessage` implementation.

I really don't agree with current solution: it is neither `NoOp`, nor `Simple`.

@garyrussell ?