check id only in current object

In our practice we use rivets as a part of framework. Working with "controllers" we use special binder which can create new scopes and inherits it from parent via `Object.create`. When scope inherits from binded object (which has `_rv` attr) adapter don't create new callbacks in weak map. Thats why we need to check `_rv` attribute via `hasOwnProperty` method

Example: http://jsfiddle.net/669ze/ (Press the second button and then the first button)