Initialize model classes upon filtering, not on class load

We are using the mutations gem in a project and experienced issues with mocking out model classes. Because the `ModelFilter` is initialized upon class load, it is not possible to first mock a model. In our unit tests, we completely mock out our ActiveRecord model:

```ruby
class MyMutation
   required do
       model :my_model
   end
end

class MyModel < ActiveRecord::Base
end

describe MyMutation do

    before do
      mock_model("MyModel")
    end

    it "..." do
       # Some tests
    end

end
```

This code would raise a `constant ActiveRecord not found` exception because mutations tries to constantize `my_model` when loading the `MyMutation` class. I think it is better to look for the model class when the actual filtering is happing, this allows for easier mocking of these classes.