Use double equals for the select widget to allow for numeric values

Because the `select` widget uses `Object.keys()`, it never reflects numerical values. 

Given the following:

```javascript
var form = forms.create({
  hours: fields.number({
    required: true,
    label: 'Hours',
    choices: _.range(24),
    widget: widgets.select()
  })
});
```

`form.bind({hours: 10}).toHTML()` will show `0` as selected.