Allow strings to be sent as collection to select

Why?

The main reason is to allow the following:

fields_for @post do |f|
  f.select(:category_id, grouped_options_for_select(...))
end

Since grouped_options_for_select returns a string, prior to this patch this wouldn't work. We would have to use select_tag method, which is not very friendly since it does not generate names properly and does not work with nested attributes as well (it wouldn't generate blog[post][0][category], unless explicitly given).