fix(datepicker): retain manually entered values even if invalid

Fixes #1710

Closes #1725

src/datepicker/datepicker-input.spec.ts

@@ -202,6 +202,33 @@ describe('NgbInputDatepicker', () => {
        }));
   });
 
+  describe('manual data entry', () => {
+
+    it('should reformat value entered by a user when it is valid', fakeAsync(() => {
+         const fixture = createTestCmpt(`<input ngbDatepicker (ngModelChange)="date">`);
+         const inputDebugEl = fixture.debugElement.query(By.css('input'));
+
+         inputDebugEl.triggerEventHandler('change', {target: {value: '2016-9-1'}});
+         tick();
+         fixture.detectChanges();
+
+         expect(inputDebugEl.nativeElement.value).toBe('2016-09-01');
+       }));
+
+    it('should retain value entered by a user if it is not valid', fakeAsync(() => {
+         const fixture = createTestCmpt(`<input ngbDatepicker (ngModelChange)="date">`);
+         const inputDebugEl = fixture.debugElement.query(By.css('input'));
+
+         inputDebugEl.nativeElement.value = '2016-09-aa';
+         inputDebugEl.triggerEventHandler('change', {target: {value: inputDebugEl.nativeElement.value}});
+         tick();
+         fixture.detectChanges();
+
+         expect(inputDebugEl.nativeElement.value).toBe('2016-09-aa');
+       }));
+
+  });
+
   describe('validation', () => {
 
     describe('values set from model', () => {

src/datepicker/datepicker-input.ts

@@ -192,7 +192,7 @@ export class NgbInputDatepicker implements OnChanges,
   manualDateChange(value: string, updateView = false) {
     this._model = this._service.toValidDate(this._parserFormatter.parse(value), null);
     this._onChange(this._model ? this._model.toStruct() : (value === '' ? null : value));
-    if (updateView) {
+    if (updateView && this._model) {
       this._writeModelValue(this._model);
     }
   }