fix(dropdown): correctly update isOpen

- Update bound value of `isOpen` correctly on toggle

Closes #5589
Fixes #3261

Author: wesleycho

src/dropdown/dropdown.js

@@ -118,7 +118,12 @@ angular.module('ui.bootstrap.dropdown', ['ui.bootstrap.position'])
   };
 
   this.toggle = function(open) {
-    return scope.isOpen = arguments.length ? !!open : !scope.isOpen;
+    scope.isOpen = arguments.length ? !!open : !scope.isOpen;
+    if (angular.isFunction(setIsOpen)) {
+      setIsOpen(scope, scope.isOpen);
+    }
+
+    return scope.isOpen;
   };
 
   // Allow other directives to watch status