fix(collapse): Prevent consecutive transitions & tidy up code

Previously, there would be a case where the two transitions would run
as the cancel would asynchronously invoke the reject handler of the
transition which would set the currTransition to undefined even when
the currTransition has been replaced with a new transition.

src/collapse/collapse.js

@@ -75,7 +75,6 @@ angular.module('ui.bootstrap.collapse', ['ui.bootstrap.transition'])
             expand();
           }
         });
-
       }
     };
   }]);

src/collapse/test/collapse.spec.js

@@ -55,6 +55,23 @@ describe('collapse directive', function () {
     expect(element.height()).not.toBe(0);
   });
 
+  it('should expand if isCollapsed = true with animation on subsequent uses', function() {
+    scope.isCollapsed = false;
+    scope.$digest();
+    scope.isCollapsed = true;
+    scope.$digest();
+    scope.isCollapsed = false;
+    scope.$digest();
+    scope.isCollapsed = true;
+    scope.$digest();
+    $timeout.flush();
+    expect(element.height()).toBe(0);
+    if ($transition.transitionEndEventName) {
+      element.triggerHandler($transition.transitionEndEventName);
+      expect(element.height()).toBe(0);
+    }
+  });
+
   describe('dynamic content', function() {
     beforeEach(function() {
       element = angular.element('<div collapse="isCollapsed"><p>Initial content</p><div ng-show="exp">Additional content</div></div>');