message: feat(isOpen): treat undefined value as default behaviour
  closes
diff: @@ -220,7 +220,7 @@ export class NgSelectComponent implements OnDestroy, OnChanges, AfterViewInit, C
  this._setItems(changes.items.currentValue || []);
  }
  if (changes.isOpen) {
  - this._manualOpen = true;
  + this._manualOpen = isDefined(changes.isOpen.currentValue);
  }
  }
  