code: public static <T> List<T> toUnique(List<T> self) {
          return toUnique(self, (Comparator<T>) null);
      }
docs: Returns a List containing the items from the List but with duplicates removed
  using the natural ordering of the items to determine uniqueness.
  <p>
  <pre class="groovyTestCase">
  def letters = ['c', 'a', 't', 's', 'a', 't', 'h', 'a', 't']
  def expected = ['c', 'a', 't', 's', 'h']
  assert letters.toUnique() == expected
  </pre>
  
  @param self       a List
  @return the List of non-duplicate items
  @since 2.4.0