instance method _set_vary_header

# File actionpack/lib/action_controller/metal/rendering.rb, line 220
      def _set_vary_header
        if request.should_apply_vary_header?
          vary_headers = response.headers["Vary"].to_s.split(",").map(&:strip).reject(&:blank?)

          unless vary_headers.include?("*")
            request.vary_headers.each do |header|
              vary_headers << header unless vary_headers.any? { |existing| existing.casecmp?(header) }
            end
            response.headers["Vary"] = vary_headers.join(", ")
          end
        end
      end