ObjectMapper should detect if the session is already opened

Please refer to [stackoverflow](http://stackoverflow.com/questions/32779557/spring-mvc-integration-test-with-hibernate-and-jackson-hibernate-mapper-illegal)

When spring integration test is annotated with @Transactional, persists a child entity and then calls a controller to return the parent entity then exception is thrown. It happens because the child entity is stored in a first session opened by @Transactional and object mapper tries to open another session to convert the parent entity to json, so the child entity appears in two sessions which lead to the exception.