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

Allows fields that are arrays of strings to be passed to `<select multiple>`
inputs.
I agree, if the select isn't `multiple`, we'd just select the last `option` provided.

Is correcting this a formulaic concern? Is it a form-filling 'failure'?

I don't think 'silently' failing will be an issue, since controllers expecting an array will be requiring `form[field][]` while controllers expecting a single option will require `form[field]`.