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

Allows fields that are arrays of strings to be passed to `<select multiple>`
inputs.
Oh right, I forgot we're passing in `label` and `value`. 

My first instinct was to use `SimpleDelegator`, and I passed in `self` instead of `label, value`. I got to the point where I made those fields public, but then had to duplicate `contains_all_options?`, so I fell back to inheritance.