instance method ensure_session_key!

# File actionpack/lib/action_dispatch/middleware/session/abstract_store.rb, line 213
        def ensure_session_key!
          if @key.blank?
            raise ArgumentError, 'A key is required to write a ' +
              'cookie containing the session data. Use ' +
              'config.session_store SESSION_STORE, { :key => ' +
              '"_myapp_session" } in config/application.rb'
          end
        end