Adding 'reload' to trackables specs

When using Mongoid 3.1.6, ruby 1.9.3 and Rails 3.2.16 I have found that ```Undo!``` and ```Redo!``` methods are not persisting when called directly on the document. For example: 

```ruby 
@book = Book.create(:name => 'name 1')
@book.update_attributes(:name => 'name 2')
@book.update_attributes(:name => 'name 3')
@book.undo! @user, :from => 3, :to => 1             #=> true
@book.name    #=> 'name 1'
@book.reload.name    #=> 'name 3'
```

However when calling these methods on the history tracks, changes are being persisted. For example: 

```ruby 
@track = @book.history_tracks.last
@track.undo!  #=> true
@book.name    #=> 'name 2'
@book.reload.name    #=> 'name 2'
```

I am not sure why this is the case, though I had a look at the specs and discovered that ```reload``` was not being called on the objects before checking if ```undo!``` or ```redo!``` worked. I've decided to add that and the specs are now failing. Is this the intended behaviour?