Add grouped_options_for_select_with_cache method to ActionView::Helpers::FormOptionsHelper in order to avoid recreate a html string options for the same collection

cached_grouped_options_for_select.rb

module ActionView
  module Helpers
    module FormOptionsHelper
      #
      # NOTE: This approach does not work with the :disabled flag and other HTML options that 
      # grouped_options_for_select_with_cache supports, it was done
      # this way in order to improve the performance of large select tags that are rebuilt 
      # more than one time per request.
      def grouped_options_for_select_with_cache(grouped_options, selected_key = nil, prompt = nil)
        @grouped_options_cache ||= {}
        object_id = grouped_options.object_id
        body = ''
        body << content_tag(:option, prompt, { :value => "" }, true) if prompt

        grouped_options = grouped_options.sort if grouped_options.is_a?(Hash)

        if @grouped_options_cache[object_id].nil? 
          @grouped_options_cache[object_id] = grouped_options.map do |group|
            content_tag(:optgroup, options_for_select(group[1], selected_key), :label => group[0])
          end
          @grouped_options_cache[object_id] = @grouped_options_cache[object_id].join('')      
          body << @grouped_options_cache[object_id]
        else
          selected_value = %(value="#{selected_key}")
          body << @grouped_options_cache[object_id].gsub(selected_value, %(selected="selected" #{selected_value}))
        end
        
        body.html_safe

      end
    end
  end
end