instance method ensure_session_key

# File actionpack/lib/action_controller/session/cookie_store.rb, line 176
        def ensure_session_key(key)
          if key.blank?
            raise ArgumentError, 'A key is required to write a ' +
              'cookie containing the session data. Use ' +
              'config.action_controller.session = { :key => ' +
              '"_myapp_session", :secret => "some secret phrase" } in ' +
              'config/environment.rb'
          end
        end