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

Allows fields that are arrays of strings to be passed to `<select multiple>`
inputs.
Looking at the [docs](http://www.ruby-doc.org/gems/docs/c/capybara-rails-2-2-0.4.1.1/Capybara/Node/Matchers.html#method-i-has_select-3F), it doesn't look like we can filter for multiple selects.

Would we really need to check for this? Wouldn't a `select`'s single/multiple selection come out when trying to fill it in and assert on the side effects of submitting the form?

Wouldn't the onus be on the caller to pass in the correct label?