instance method unmarshal

# File actionpack/lib/action_controller/session/cookie_store.rb, line 138
    def unmarshal(cookie)
      if cookie
        data, digest = cookie.split('--')

        # Do two checks to transparently support old double-escaped data.
        unless secure_compare(digest, generate_digest(data)) || secure_compare(digest, generate_digest(data = CGI.unescape(data)))
          delete
          raise TamperedWithCookie
        end

        Marshal.load(ActiveSupport::Base64.decode64(data))
      end
    end