Automation operation should use core-io marshaller when available

An automation operation that return a custom type object will have this object serialized using the DefaultJsonAdapter.

When Writers (core-io marshaller) are available for the returned type they should be used instead.

The code that choose the DefaultJsonMarshaller or core-io marshaller is done in automation-server:
org.nuxeo.ecm.automation.server.jaxrs.ResponseHelper#getResponse(java.lang.Object, javax.servlet.http.HttpServletRequest, int)

Note that for an operation that returns a DocumentModel the core-io marshallers is used.