DecimalRangeField and IntegerRangeField field types do not render the value

When using DecimalRangeField and IntegerRangeField data types, the actual value that the field is set to (in the resulting slider) is never rendered. This behavior is also present in your own official example:

http://173.212.227.186/form

```
integerslider = IntegerRangeField(render_kw={'min': '0', 'max': '4'})
```

Is there some undocumented way to render the value of the slider? Otherwise, its confusing to know what value is being submitted, especially for a large range of values.