@Test
public void filterValueThatShouldBeReadFromExternal() {

    final DittoHeaderDefinition headerDefinition = DittoHeaderDefinition.CORRELATION_ID;
    final String value = "correlation-id";
    final Map<String, HeaderDefinition> headerDefinitions =
            Maps.newHashMap(headerDefinition.getKey(), headerDefinition);
    final CheckExternalFilter underTest = CheckExternalFilter.shouldReadFromExternal(headerDefinitions);
    assertThat(underTest.apply(headerDefinition.getKey(), value)).isEqualTo(value);
}