fix(modal): allow modal.{dismiss,close} to be called again

$modalStack.openWindows.get(modalInstance) is undefined after the first dismiss and throws:
-   TypeError: Cannot read property 'value' of undefined

Closes #1972

src/modal/modal.js

@@ -245,17 +245,17 @@ angular.module('ui.bootstrap.modal', ['ui.bootstrap.transition'])
       };
 
       $modalStack.close = function (modalInstance, result) {
-        var modalWindow = openedWindows.get(modalInstance).value;
+        var modalWindow = openedWindows.get(modalInstance);
         if (modalWindow) {
-          modalWindow.deferred.resolve(result);
+          modalWindow.value.deferred.resolve(result);
           removeModalWindow(modalInstance);
         }
       };
 
       $modalStack.dismiss = function (modalInstance, reason) {
-        var modalWindow = openedWindows.get(modalInstance).value;
+        var modalWindow = openedWindows.get(modalInstance);
         if (modalWindow) {
-          modalWindow.deferred.reject(reason);
+          modalWindow.value.deferred.reject(reason);
           removeModalWindow(modalInstance);
         }
       };

src/modal/test/modal.spec.js

@@ -108,6 +108,7 @@ describe('$modal', function () {
 
   function close(modal, result) {
     modal.close(result);
+    $timeout.flush();
     $rootScope.$digest();
   }
 
@@ -135,6 +136,36 @@ describe('$modal', function () {
       expect($document).not.toHaveBackdrop();
     });
 
+    it('should not throw an exception on a second dismiss', function () {
+
+      var modal = open({template: '<div>Content</div>'});
+
+      expect($document).toHaveModalsOpen(1);
+      expect($document).toHaveModalOpenWithContent('Content', 'div');
+      expect($document).toHaveBackdrop();
+
+      dismiss(modal, 'closing in test');
+
+      expect($document).toHaveModalsOpen(0);
+
+      dismiss(modal, 'closing in test');
+    });
+
+    it('should not throw an exception on a second close', function () {
+
+      var modal = open({template: '<div>Content</div>'});
+
+      expect($document).toHaveModalsOpen(1);
+      expect($document).toHaveModalOpenWithContent('Content', 'div');
+      expect($document).toHaveBackdrop();
+
+      close(modal, 'closing in test');
+
+      expect($document).toHaveModalsOpen(0);
+
+      close(modal, 'closing in test');
+    });
+
     it('should open a modal from templateUrl', function () {
 
       $templateCache.put('content.html', '<div>URL Content</div>');