@Override
  @Nullable
  public V put(@Nullable K key, @Nullable V value) {
    return put(key, value, true);
  }
@Test
  void shouldGetKeySet() {
    this.map.put(123, "123");
    this.map.put(456, null);
    this.map.put(null, "789");
    Set<Integer> expected = new HashSet<>();
    expected.add(123);
    expected.add(456);
    expected.add(null);
    assertThat(this.map.keySet(), equalTo(expected));
  }