restore will not remove properties that were added since the last backup

restore will not remove properties that were added since the last backup

Per http://canjs.com/docs/can.Map.backup.prototype.restore.html

This is never the behavior I would want from backup/restore.  In restore, `this._attrs(props);` could be called with 'true' as the second argument to remove new properties, and that could be an optional attribute on restore.