Serialize checked radio with value==="false" to boolean false

Makes the following HTML to be serialized as `{fixed: false}`:

```html
  <input type="radio" value="true"  name="fixed" /> Fixed
  <input type="radio" value="false" name="fixed" checked="checked"/> Variable
```

Without this commit `form2js` would serialize the frist to `{fixed: true}` and the latter to `{fixed: "false"}` causing confusion with tests:

```js
var obj = form2js(form);
if (obj.fixed) {
   console.log("This string will be logged to console.. no matter what");
}
```