@Test
 public void canGetAllAttributeNamesByNonExistingNamespace() {
   List<String> names = element.attrNames(NON_EXISTING);
   assertThat(names).isEmpty();
 }
 @Test
 public void canGetAllAttributeNamesByNamespace() {
   List<String> names = element.attrNames(EXAMPLE_NAMESPACE);
   assertThat(names).containsOnly("dueUntil");
 }
 @Test
 public void canGetAllAttributeNamesByNullNamespace() {
   List<String> names = element.attrNames(null);
   assertThat(names).containsOnly("order");
 }
 @Test
 public void canGetAllAttributeNames() {
   List<String> names = element.attrNames();
   assertThat(names).containsOnly("order", "dueUntil", "xmlns", "ex");
 }