title: ngModel not updated in ng-select component
body: <p>I have ng-select component in my code and it initializes as below in HTML. </p>
  
  <pre><code>&lt;ng-select id="SelectType" placeholder="Student" [multiple]="true"
                [items]="students" groupBy="type" [selectableGroup]="false" [(ngModel)]="selectedstudents"
                [closeOnSelect]="false" bindLabel="name" (add)="onSelectFirstDropdown($event)"
                (remove)="onRemoveFirstDropdown($event.value)"&gt;
  </code></pre>
  
  <p>I have select multiple students from the dropdown and after doing some modifications in the ts file I want to remove one student in "selectedstudents" List. But when I remove the item from the list it's not updated to the ng-select and item still remain selected in the dropdown. How can I solve this issue? 
  Thank you</p>