fix(ng-select): isOpen is optional as noted in the README (#2322)

Co-authored-by: Pavan Kumar Jadda <17564080+pavankjadda@users.noreply.github.com>

README.md

@@ -193,7 +193,7 @@ map: {
 | loadingText | `string` | `Loading...` | no | Set custom text when for loading items |
 | labelForId | `string` | `-` | no | Id to associate control with label. |
 | [markFirst] | `boolean` | `true` | no | Marks first item as focused when opening/filtering. |
-| [isOpen] | `boolean` | `-` | no | Allows manual control of dropdown opening and closing. `True` - won't close. `False` - won't open. |
+| [isOpen] | `boolean` | `-` | no | Allows manual control of dropdown opening and closing. `true` - won't close. `false` - won't open. |
 | maxSelectedItems | `number` | none | no | When multiple = true, allows to set a limit number of selection. |
 | [hideSelected] | `boolean` | `false` | no | Allows to hide selected items. |
 | [multiple] | `boolean` | `false` | no | Allows to select multiple items. |

src/ng-select/lib/ng-select.component.ts

@@ -113,7 +113,7 @@ export class NgSelectComponent implements OnDestroy, OnChanges, OnInit, AfterVie
     @Input() @HostBinding('class.ng-select-taggable') addTag: boolean | AddTagFn = false;
     @Input() @HostBinding('class.ng-select-searchable') searchable = true;
     @Input() @HostBinding('class.ng-select-clearable') clearable = true;
-    @Input() @HostBinding('class.ng-select-opened') isOpen = false;
+    @Input() @HostBinding('class.ng-select-opened') isOpen?: boolean = false;
 
     @Input()
     get items() { return this._items };