allow form inputs to have names of "className"

The way browsers map inputs to form properties, it is impossible to access the class name of a `<form>` element containing an `<input>` with the attribute of `name="className"` as `form.className` maps to the `<input name="className">` rather than the `class` attribute of the `<form>`.

This causes undesired results when using the current `addClass()` implementation:

```js
var form = jQuery('<form class="a">');
form.append('<input name="className">');
form.addClass('b');

assert(form.getAttribute('class') == 'a b');
// Error: Expected "[HTMLInputElement] b" to equal "a b".
```