Fallback to `select` if array of `checkboxes` fail

Allows fields that are arrays of strings to be passed to `<select multiple>`
inputs.
Hate to complicate it for you, but we'd want the same behavior for the `select` if it doesn't have a given option.

```
if has_multi_select?
  select_options
else
  check_boxes
end
rescue NoSuchOption, NoSuchCheckbox
  raise InputNotFound
```