solves default-scope issue and some methods

When a ```default scope``` is defined in a model and I was calling an ```undo! @user``` on an history-tracked object, I was getting the error: 
```NoMethodError: undefined method `history_trackable_options' for nil:NilClass```

I tracked this issue down to the ```traverse_association_chain``` method in history/tracker.rb, which is indirectly called in the ```undo!``` and ```redo!``` methods. I simply added an unscoped to remove the 'default scope' that was being applied and the error was no longer occurring. Even when there is no default scope defined, ```unscoped``` will still work and returns all the documents for that class without any filtering. 

What are your thoughts on this matter?