Does coerce still work?

I add this to my api.rb

```
use Goliath::Rack::Validation::CoerceValue, :key => 'url', :as => ValidUrl
```

And required this class   /libs/valid_url.rb in my runner.rb.

```
class ValidUrl

  def coerce value, opts ={}
    raise Goliath::Rack::Validation::FailedCoerce.new([400, {}, "URL is invalid"])
  end
end
```

The exception is never thrown. Am I missing something else?
