feat(datepicker): watch for changes when falsy

- When initial value is falsy, continue to watch for changes to update

Closes #5672
Closes #5677

Author: wesleycho

Diff for: src/datepicker/datepicker.js

@@ -94,23 +94,21 @@ angular.module('ui.bootstrap.datepicker', ['ui.bootstrap.dateparser', 'ui.bootst
         break;
       case 'maxDate':
       case 'minDate':
-        if ($scope.datepickerOptions[key]) {
-          $scope.$watch('datepickerOptions.' + key, function(value) {
-            if (value) {
-              if (angular.isDate(value)) {
-                self[key] = dateParser.fromTimezone(new Date(value), ngModelOptions.timezone);
-              } else {
-                self[key] = new Date(dateFilter(value, 'medium'));
-              }
+        $scope.$watch('datepickerOptions.' + key, function(value) {
+          if (value) {
+            if (angular.isDate(value)) {
+              self[key] = dateParser.fromTimezone(new Date(value), ngModelOptions.timezone);
             } else {
-              self[key] = null;
+              self[key] = new Date(dateFilter(value, 'medium'));
             }
+          } else {
+            self[key] = datepickerConfig[key] ?
+              dateParser.fromTimezone(new Date(datepickerConfig[key]), ngModelOptions.timezone) :
+              null;
+          }
 
-            self.refreshView();
-          });
-        } else {
-          self[key] = datepickerConfig[key] ? dateParser.fromTimezone(new Date(datepickerConfig[key]), ngModelOptions.timezone) : null;
-        }
+          self.refreshView();
+        });
 
         break;
       case 'maxMode':

Diff for: src/datepicker/test/datepicker.spec.js

@@ -920,6 +920,30 @@ describe('datepicker', function() {
         });
       });
 
+      describe('minDate with no initial value', function() {
+        beforeEach(function() {
+          $rootScope.options = {};
+          $rootScope.date = new Date('September 10, 2010');
+          element = $compile('<uib-datepicker ng-model="date" datepicker-options="options"></uib-datepicker>')($rootScope);
+          $rootScope.$digest();
+        });
+
+        it('should toggle appropriately', function() {
+          var buttons = getAllOptionsEl();
+          angular.forEach(buttons, function(button, index) {
+            expect(angular.element(button).prop('disabled')).toBe(false);
+          });
+
+          $rootScope.options.minDate = new Date('September 12, 2010');
+          $rootScope.$digest();
+
+          refreshedButtons = getAllOptionsEl();
+          angular.forEach(refreshedButtons, function(button, index) {
+            expect(angular.element(button).prop('disabled')).toBe(index < 14);
+          });
+        });
+      });
+
       describe('minDate', function() {
         beforeEach(function() {
           $rootScope.options = {
@@ -1019,6 +1043,30 @@ describe('datepicker', function() {
         });
       });
 
+      describe('maxDate with no initial value', function() {
+        beforeEach(function() {
+          $rootScope.options = {};
+          $rootScope.date = new Date('September 10, 2010');
+          element = $compile('<uib-datepicker ng-model="date" datepicker-options="options"></uib-datepicker>')($rootScope);
+          $rootScope.$digest();
+        });
+
+        it('should toggle appropriately', function() {
+          var buttons = getAllOptionsEl();
+          angular.forEach(buttons, function(button, index) {
+            expect(angular.element(button).prop('disabled')).toBe(false);
+          });
+
+          $rootScope.options.maxDate = new Date('September 25, 2010');
+          $rootScope.$digest();
+
+          refreshedButtons = getAllOptionsEl();
+          angular.forEach(refreshedButtons, function(button, index) {
+            expect(angular.element(button).prop('disabled')).toBe(index > 27);
+          });
+        });
+      });
+
       describe('maxDate', function() {
         beforeEach(function() {
           $rootScope.options = {