Fixes human_attribute_name in Label

Fix `humanize_attribute_name`

This is the code that was broken. Passing in `class_name` but using `model_name` in the method. `#classify` will also raise an error if `model_name` is not found.

```ruby
def class_exists?(class_name)
  Object.const_defined?(model_name.to_s.classify)
rescue NameError
  false
end
```
@CraigWilliams correct me if I'm wrong, but this whole change can be simplified to changing this line to `class_name` and adding the tests.