Use last item from array when mapping to a non array/slice

When multiple values are posted from a form, it makes sense to use the last defined value, which would be from the last defined element.

This allow overriding the value, which is specially useful when dealing with checkboxes, to have a default value submitted when the checkbox is not selected.

```html
<input name="foo" type="hidden" value="0">
<input name="foo" type="checkbox" value="1">
```