Responsebody is empty after finished test controller

My request has a body and test is finished, but I need test response body but this is empty, why ???

@RunWith(SpringRunner.class)
@WebMvcTest(RentalController.class)
public class RentalControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private RentalService rentalService;

    @MockBean
    private RentalMapper rentalMapper;

    @Test
    public void shouldReturnRentalByUsername() throws Exception {

        User user = new User(1L,"janek22", "janek", "janek2@wp.pl");
        Bike bike = new Bike("AB123", false);
        LocalDateTime dateTime = LocalDateTime.of(2019,5,24,1,0,0);
        Rental rental = new Rental(user, bike, dateTime , dateTime,
                "22.222", "33.444", "33.444", "33.444", "33.444" );
        given(rentalService.findByUsername(anyString())).willReturn(Collections.singletonList(rental));

        ObjectMapper objectMapper = new ObjectMapper();
        String content = objectMapper.writeValueAsString(user);

        mockMvc.perform(get("/rental/" + user.getUsername())
                    .contentType(MediaType.APPLICATION_PROBLEM_JSON_UTF8_VALUE)
                    .content(content))
                .andExpect(status().isOk())
                .andDo(print());



    }
}

MockHttpServletResponse:
Status = 200
Error message = null
Headers = [Content-Type:"application/json;charset=UTF-8"]
Content type = application/json;charset=UTF-8
Body = []
Forwarded URL = null
Redirected URL = null
Cookies = []