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
```
@jessieay What is it about dynamic dispatch that you find icky? Is it that the class name is capitalized? This is avoiding type checking. Here is a quote from page 1 of Ruby Science.

>  A better solution is to take advantage of Ruby’s
ability to invoke different behavior based on an object’s class, called “dynamic
dispatch.” Using a case statement with a type code inelegantly reproduces
dynamic dispatch.