CuppaLabs#613 Fix: Cannot add property 0, object is not extensible

Solution: Cloning the array together with the new value.

projects/angular2-multiselect-dropdown-lib/src/lib/multiselect.component.ts

@@ -377,7 +377,7 @@ export class AngularMultiSelect implements OnInit, ControlValueAccessor, OnChang
             this.closeDropdown();
         }
         else
-            this.selectedItems.push(item);
+            this.selectedItems = [...this.selectedItems, item];
         this.onChangeCallback(this.selectedItems);
         this.onTouchedCallback(this.selectedItems);
     }