Unable to add className to element using widgetAttr

First off I really enjoying using this library and thanks for making it. I did however come across an issue when trying to add an additional class to form element. using `widgetAttr`.

For example:

```
advancedSettings: ->
upc: CharField widgetAttrs:
placeholder: "Enter Text Here"
className: "test-class"
taxes: ChoiceField({choices: @choices})
modifiers: ChoiceField({choices: @moreChoices})
allowReturns: BooleanField({required: false})
```

When this renders, the form only has classes from newform-bootstrap and my `test-class` is not applied. Any ideas whats happening?

Thank-you,