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
```
I don't know if I follow the question. The current (and unchanged) behavior is strings passed into `fill_form` as an attribute key are passed to `fill_in` unchanged, and for symbols we try to translate, then fall back to AR::Base to give us a humanized version, then just make it a string and pass it along.

So if you did: `fill_form(:user, t('user.name') => 'Caleb')` and `t('user.name')` returned `"Name"`, that would be equivalent to `fill_form(:user, "Name" => 'Caleb')` not `fill_form(:user, t("Name", scope: ...) => 'Caleb')`