fix(datepickerPopup): clear date when button is clicked

- Fix clearing the date when the clear button is clicked

Closes #5945
Fixes #5906

src/datepickerPopup/popup.js

@@ -210,9 +210,7 @@ function($scope, $element, $attrs, $compile, $log, $parse, $window, $document, $
 
   // Inner change
   $scope.dateSelection = function(dt) {
-    if (angular.isDefined(dt)) {
-      $scope.date = dt;
-    }
+    $scope.date = dt;
     var date = $scope.date ? dateParser.filter($scope.date, dateFormat) : null; // Setting to NULL is necessary for form validators to function
     $element.val(date);
     ngModel.$setViewValue(date);

src/datepickerPopup/test/popup.spec.js

@@ -1041,6 +1041,13 @@ describe('datepicker popup', function() {
         expect($rootScope.date).toBe(null);
       });
 
+      it('should clear the previously selected date', function() {
+         $rootScope.date = new Date();
+         $rootScope.$digest();
+         buttons.eq(1).click();
+         expect($rootScope.date).toBe(null);
+      });
+
       it('should have a button to close calendar', function() {
         buttons.eq(2).click();
         assignElements(wrapElement);