@Test
public void nullBirthDay() throws Exception {

    SafeCalendar fromJson = validBirthdayAdapter().fromJson("{\n"
          + "  \"year\":null,\n"
          + "  \"month\":null,\n"
          + "  \"day\":null\n"
          + '}');
    assertThat(fromJson.isSet(Calendar.YEAR)).isFalse();
    assertThat(fromJson.isSet(Calendar.MONTH)).isFalse();
    assertThat(fromJson.isSet(Calendar.DAY_OF_MONTH)).isFalse();
}