fix(datepicker): dereference date initialization

- Dereference the date set to active date to prevent model automatically
  updating from equal reference

Closes #5643
Fixes #5441

Author: wesleycho

src/datepicker/datepicker.js

@@ -271,7 +271,9 @@ angular.module('ui.bootstrap.datepicker', ['ui.bootstrap.dateparser', 'ui.bootst
     ngModelCtrl = ngModelCtrl_;
     ngModelOptions = ngModelCtrl_.$options || datepickerConfig.ngModelOptions;
 
-    this.activeDate = ngModelCtrl.$modelValue || new Date();
+    this.activeDate = ngModelCtrl.$modelValue ?
+      dateParser.fromTimezone(new Date(ngModelCtrl.$modelValue), ngModelOptions.timezone) :
+      dateParser.fromTimezone(new Date(), ngModelOptions.timezone);
 
     ngModelCtrl.$render = function() {
       self.render();

src/datepicker/test/datepicker.spec.js

@@ -1828,6 +1828,13 @@ describe('datepicker', function() {
         expect(getTitle()).toBe('January 2014');
       });
 
+      it('should not change model when going to next month - #5441', function() {
+        $rootScope.date = new Date('January 30, 2014');
+        $rootScope.$digest();
+        clickNextButton();
+        expect($rootScope.date).toEqual(new Date('January 30, 2014'));
+      });
+
       describe('when `model` changes', function() {
         function testCalendar() {
           expect(getTitle()).toBe('November 2005');