Fix uniqueness client side hash when using inherit

When add an uniqueness validator, validator will store the klass where the validator defined:

https://github.com/mongoid/mongoid/blob/cb86ec4a75712e6908db4ca3eda41cdf6105a331/lib/mongoid/validatable/uniqueness.rb#L29

Then use klass to query uniqueness status:

https://github.com/mongoid/mongoid/blob/cb86ec4a75712e6908db4ca3eda41cdf6105a331/lib/mongoid/validatable/uniqueness.rb#L271

It means when there are two model:

```ruby
class Book
   field :name
   validates :name, :uniqueness => true
end

class Book2 < Book; end
```

It will always use Book to check if the name is uniqueness, and that's what I expected.

So I add class name to client side hash to sync with mongoid's behavior.