Strange behavior with SpringBoot unit testing and Jackson json manipulation

I have a rest controller implemented in Java using Spring Boot. It returns an entity, and I tested this controller using PostMan. Result is a nice json of the entity (status code is 200). Now, when I run the unit test the status code is 500. I removed from the entity the annotation @JsonSerialize used in a Date field, i.e.:

@JsonSerialize(using = JsonDateSerializer.class)
private Date adate;

where the serializer is:

@Component
public class JsonDateSerializer extends JsonSerializer<Date> {

    private static final SimpleDateFormat DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd");

    @Override
    public void serialize(final Date date, final JsonGenerator gen, final SerializerProvider provider) throws IOException {
        gen.writeString(DATE_FORMAT.format(date));
    }
}

And now I get a status code 200 in the test.

The test method relies on the starter-test module of Spring boot and is:

@Test
public void test(){
    mockMvc.perform(get("/rest/path"))
                    .andExpect(status().isOk())
                    .andExpect(jsonPath("$", hasSize(1)))
                    .andExpect(jsonPath("$[0].id", is(1)))
                  .andExpect(jsonPath("$[0].adate", is("2015/01/01")));
}

When the annotation is on, the exception is:

015-08-27 22:43:17.502  WARN 8680 --- [           main] .w.s.m.s.DefaultHandlerExceptionResolver : Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: Could not write content: (was java.lang.UnsupportedOperationException) (through reference chain: com.google.common.collect.SingletonImmutableList[0]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.UnsupportedOperationException) (through reference chain: com.google.common.collect.SingletonImmutableList[0])
2015-08-27 22:43:17.504  WARN 8680 --- [           main] .w.s.m.s.DefaultHandlerExceptionResolver : Handler execution resulted in exception: Could not write content: (was java.lang.UnsupportedOperationException) (through reference chain: com.google.common.collect.SingletonImmutableList[0]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.UnsupportedOperationException) (through reference chain: com.google.common.collect.SingletonImmutableList[0])

java.lang.AssertionError: Status 
Expected :200
Actual   :500
 <Click to see difference>
    at org.springframework.test.util.AssertionErrors.fail(AssertionErrors.java:60)
    at org.springframework.test.util.AssertionErrors.assertEquals(AssertionErrors.java:89)
    at org.springframework.test.web.servlet.result.StatusResultMatchers$10.match(StatusResultMatchers.java:655)
    at org.springframework.test.web.servlet.MockMvc$1.andExpect(MockMvc.java:166)
    at com.rest.RestControllerTest.test(RestControllerTest.java:87)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:74)
    at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:85)
    at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:86)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:241)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:87)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
    at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:180)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:78)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:212)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:68)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

Pom looks like:

    <parent>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-parent</artifactId>
      <version>1.3.0.M3</version>
      <relativePath/>
    </parent>

with dependency:

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

Why that annotation doesn't work? How can I fix the unit test?