I created simple Spring Boot application with RestController and provided two test classes. One using MockMvc and one using RestAssured. All is available in referenced git repository, commit: 5e582670001373804778abd41039cf23ba476bb9.
Each class have 2 tests. One for http 200 status code and one for 415. The problem is that I'm not able to check returned content type and body in case http 415 code as MockMvc claims that there is no content type set and body doesn't contain expected "status" field.