fix(typeahead): release references on destruction

- Release reference to manually managed scopes, which prevent proper garbage collection

Closes #4299
Fixes #4298

Author: yoann-antoviaque

src/dropdown/dropdown.js

@@ -230,9 +230,10 @@ angular.module('ui.bootstrap.dropdown', ['ui.bootstrap.position'])
     }
   });
 
-  $scope.$on('$destroy', function() {
+  var offDestroy = $scope.$on('$destroy', function() {
     scope.$destroy();
   });
+  scope.$on('$destroy', offDestroy);
 }])
 
 .directive('dropdown', function() {

src/typeahead/typeahead.js

@@ -89,9 +89,10 @@ angular.module('ui.bootstrap.typeahead', ['ui.bootstrap.position'])
         //create a child scope for the typeahead directive so we are not polluting original scope
         //with typeahead-specific data (matches, query etc.)
         var scope = originalScope.$new();
-        originalScope.$on('$destroy', function() {
-          scope.$destroy();
+        var offDestroy = originalScope.$on('$destroy', function(){
+			scope.$destroy();
         });
+        scope.$on('$destroy', offDestroy);
 
         // WAI-ARIA
         var popupId = 'typeahead-' + scope.$id + '-' + Math.floor(Math.random() * 10000);