Testing with MockMvc into SpringBoot

I have a problem with MockMvc into Spring (with Spring Boot). I have a small code to learn Testing

import static org.hamcrest.Matchers.containsString;
import static     org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.print;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.test.web.servlet.MockMvc;

@RunWith(SpringRunner.class)
@SpringBootTest
@AutoConfigureMockMvc
public class ApplicationTest04_ownServer {

@Autowired
private MockMvc mockMvc;

@Test
public void shouldReturnDefaultMessage() throws Exception {
    this.mockMvc
    .perform(get("/"))
    .andDo(print())
    .andExpect(status().isOk())
    .andExpect(content().string(containsString("Hello")));
}

OK. Mi pom.xml is this

[...]
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>1.5.19.RELEASE</version>
    <relativePath /> <!-- lookup parent from repository -->
</parent>
<groupId>org.springframework</groupId>
<artifactId>Prueba</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>war</packaging>
<name>Prueba</name>

<properties>
    <java.version>1.8</java.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-devtools</artifactId>
        <scope>runtime</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-tomcat</artifactId>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>compile</scope>
    </dependency>

</dependencies>

I have several test. The works, but I have a problem with test with Mock Object or similar. For example, in the test, the controller response me a text.

@Controller
public class HomeController {

    @RequestMapping("/")
    public @ResponseBody String greeting() {
        return "Hello";
    }

}

In the test (the first code above), this is the code

@Test
public void shouldReturnDefaultMessage() throws Exception {
    this.mockMvc
    .perform(get("/"))
    .andDo(print())
    .andExpect(status().isOk())
    .andExpect(content().string(containsString("Hello")));
}

So, I expect that the response of controller was the same that the response of the test ("hello") but the Junit test is wrong.

java.lang.SecurityException: class "org.hamcrest.Matchers"'s signer information does not match 

I print the result

MockHttpServletRequest:
      HTTP Method = GET
      Request URI = /
       Parameters = {}
          Headers = {}

Handler:
             Type = hello.HomeController
           Method = public java.lang.String hello.HomeController.greeting()

Async:
    Async started = false
     Async result = null

Resolved Exception:
             Type = null

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

FlashMap:
       Attributes = null

MockHttpServletResponse:
           Status = 200
    Error message = null
         Headers = {Content-Type=[text/plain;charset=UTF-8], Content-Length=[5]}
     Content type = text/plain;charset=UTF-8
             Body = Hello
    Forwarded URL = null
   Redirected URL = null
          Cookies = []

MockHttpServletRequest:
      HTTP Method = GET
      Request URI = /
       Parameters = {}
          Headers = {}

Handler:
             Type = hello.HomeController
           Method = public java.lang.String hello.HomeController.greeting()

Async:
    Async started = false
     Async result = null

Resolved Exception:
             Type = null

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

FlashMap:
       Attributes = null

MockHttpServletResponse:
           Status = 200
    Error message = null
          Headers = {Content-Type=[text/plain;charset=UTF-8], Content-Length=[5]}
     Content type = text/plain;charset=UTF-8
             Body = Hello
    Forwarded URL = null
   Redirected URL = null
          Cookies = []

The body response is hello, isn`t is? Any idea?

Thanks in advance