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

Allows fields that are arrays of strings to be passed to `<select multiple>`
inputs.
[has_selector?](http://www.ruby-doc.org/gems/docs/c/capybara-rails-2-2-0.4.1.1/Capybara/Node/Matchers.html#method-i-has_selector-3F) should take attribute selectors: `has_selector? "select[multiple]"`