fix(datepicker): dereference date initialization

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

Author: wesleycho

Diff for: 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();