fix(flatpickr-adapter): handle missing formatter property

Restores compatibility when private component is used directly.

Author: vicgeralds

src/components/date-picker/flatpickr-adapter/flatpickr-adapter.tsx

@@ -136,7 +136,9 @@ export class DatePickerCalendar {
                 break;
         }
 
-        this.picker.formatter = this.formatter;
+        if (this.formatter) {
+            this.picker.formatter = this.formatter;
+        }
     }
 
     public componentDidUpdate() {

src/components/date-picker/pickers/Picker.ts

@@ -14,7 +14,8 @@ import { isAndroidDevice, isIOSDevice } from '../../../util/device';
 const ARIA_DATE_FORMAT = 'F j, Y';
 
 export abstract class Picker {
-    public formatter: (date: Date) => string;
+    public formatter = (date: Date) =>
+        moment(date).locale(this.getMomentLang()).format(this.dateFormat);
 
     protected dateFormat: string;
     protected language: string = 'en';