fix(datepicker): fix accidental global

- Fix accidental global usage

Closes #6188

Author: thorn0

src/datepicker/datepicker.js

@@ -126,7 +126,7 @@ angular.module('ui.bootstrap.datepicker', ['ui.bootstrap.dateparser', 'ui.bootst
       case 'minMode':
         if ($scope.datepickerOptions[key]) {
           $scope.$watch(function() { return $scope.datepickerOptions[key]; }, function(value) {
-            self[key] = $scope[key] = angular.isDefined(value) ? value : datepickerOptions[key];
+            self[key] = $scope[key] = angular.isDefined(value) ? value : $scope.datepickerOptions[key];
             if (key === 'minMode' && self.modes.indexOf($scope.datepickerOptions.datepickerMode) < self.modes.indexOf(self[key]) ||
               key === 'maxMode' && self.modes.indexOf($scope.datepickerOptions.datepickerMode) > self.modes.indexOf(self[key])) {
               $scope.datepickerMode = self[key];