title: ClassNotFoundException: org.junit.Assert exception when using MockRestServiceServer
body: <p>I'm trying to run the following test</p>
  
  <pre><code>  import org.springframework.test.web.client.MockRestServiceServer;
      ....            
  
  @Test
  void successPost () {
  MockRestServiceServer server = MockRestServiceServer.bindTo(restTemplate).build();
  
      server.expect(once(), requestTo("http://localhost:8080/test"))
                  .andExpect(header("X-AUTH", "myToken"))
                  .andRespond(withSuccess("{ \"msg\":\"hi\"}", MediaType.APPLICATION_JSON));
  
              WebClient&lt;String, DummyResponse&gt; client = new WebClient&lt;&gt;(restTemplate, retryTemplate, DummyResponse.class);
              DummyResponse result = client.post("http://localhost:8080","/test", "myRequest", "myToken");
  
              // Verify all expectations met
              server.verify();
              assertThat(result.getMsg(), is("hi"));
  }
  </code></pre>
  
  <p>and it fails with  when I have <code>.andExpect(header("X-AUTH", "myToken"))</code>
  ClassNotFoundException: org.junit.Assert</p>
  
  <p>Project contains only JUnit 5 dependencies and </p>
  
  <pre><code>&lt;dependency&gt;
          &lt;groupId&gt;org.springframework.boot&lt;/groupId&gt;
          &lt;artifactId&gt;spring-boot-starter-test&lt;/artifactId&gt;
          &lt;version&gt;2.1.0.RELEASE&lt;/version&gt;
          &lt;scope&gt;test&lt;/scope&gt;
          &lt;exclusions&gt;
              &lt;exclusion&gt;
                  &lt;groupId&gt;junit&lt;/groupId&gt;
                  &lt;artifactId&gt;junit&lt;/artifactId&gt;
              &lt;/exclusion&gt;
          &lt;/exclusions&gt;
      &lt;/dependency&gt;
  </code></pre>