Actions should not marshal into JSON if the input was not JSON

[nicolaferraro]

They should deserialize JSON if the content-type is "application/json" and then marshal it again into JSON before completing. But if the input is not JSON, they should leave it as JsonNode.

In other words, this step should be done only if the flow has entered the application/json choice branch before (we may set an exchange property to track).

Also setting the content-type should be conditional.