Fallback to `simple_form` DOM `id` if `I18n` label not found

If no input can be found with the `simple_form` label, fallback to `simple_form` ID

```ruby
fill_form(:post, {title: "Formulaic", tag: "capybara"})
```

Would fill in `#post_title` with "Formulaic", and `#post_tag` with "Capybara"