"Content type not set" when Unit Testing Spring RESTful controller

I have a Rest controller similar to this one:

@RestController
public class UserRestController {

    @Autowired
    private UserService userService;

    @RequestMapping(value = "/user/activate", method = RequestMethod.POST)
    public ResponseEntity<UserDTO> activate(
            @RequestParam(required = true) final String email,
            @RequestParam(required = true) final String key) {

        UserDTO userDTO = userService.activateAccount(email, key);
        return new ResponseEntity<UserDTO>(userDTO, HttpStatus.OK);
    }
}

When I invoke it using Postman and I don't send the 'key' parameter, I receive this JSON message:

{
    "timestamp": 1446211575193,
    "status": 400,
    "error": "Bad Request",
    "exception": "org.springframework.web.bind.MissingServletRequestParameterException",
    "message": "Required String parameter 'key' is not present",
    "path": "/user/activate"
}

On the other hand, I am testing this method with JUnit and the MockMVC Spring utility.

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = ApplicationConfig.class)
@WebAppConfiguration
public class UserRestControllerTest {

    private static MockMvc mockMvc;

    @Mock
    private UserService userService;

    @InjectMocks
    private UserRestController userRestController;

    @Before
    public void setup() {

        MockitoAnnotations.initMocks(this);
        mockMvc = MockMvcBuilders
                .standaloneSetup(userRestController)
                .setMessageConverters(
                        new MappingJackson2HttpMessageConverter(),
                        new Jaxb2RootElementHttpMessageConverter()).build();

    }

    @Test
    public void testActivateRequiredParams() throws Exception {

        mockMvc.perform(
                MockMvcRequestBuilders.post("/user/activate")
                .contentType(MediaType.APPLICATION_FORM_URLENCODED)
                .accept(MediaType.APPLICATION_JSON))
                .andDo(MockMvcResultHandlers.print())
                .andExpect(MockMvcResultMatchers.status().isBadRequest())
                .andExpect(
                        MockMvcResultMatchers.content().contentType(
                                UtilsUnitTest.APPLICATION_JSON_UTF8))
                .andExpect(
                        jsonPath(
                                "message",
                                is("Required String parameter 'email' is not present")));

     }
}

But when I execute this test I notice that the response is not a JSON message. In fact, I get an exception:

java.lang.AssertionError: Content type not set

Particularly, the completed result is

MockHttpServletRequest:
         HTTP Method = POST
         Request URI = /user/activate
          Parameters = {}
             Headers = {Content-Type=[application/x-www-form-urlencoded]}

             Handler:
                Type = com.company.controller.UserRestController
              Method = public org.springframework.http.ResponseEntity<com.company.dto.UserDTO> com.company.controller.UserRestController.activate(java.lang.String,java.lang.String)

               Async:
       Async started = false
        Async result = null

  Resolved Exception:
                Type = org.springframework.web.bind.MissingServletRequestParameterException

        ModelAndView:
           View name = null
                View = null
               Model = null

            FlashMap:

MockHttpServletResponse:
              Status = 400
       Error message = Required String parameter 'email' is not present
             Headers = {}
        Content type = null
                Body = 
       Forwarded URL = null
      Redirected URL = null
             Cookies = []

I deduce that when an exception is thrown this is not converted to JSON format (I get a correct JSON result when I send the email and key parameters)

The question is obvious: what should I change in the Unit Test configuration to get a JSON error message in case of an exception be thrown?