module ActionDispatch::Integration::Runner

def method_missing(method, *args, &block)

Delegate unhandled messages to the current session instance.
def method_missing(method, *args, &block)
  if integration_session.respond_to?(method)
    integration_session.public_send(method, *args, &block).tap do
      copy_session_variables!
    end
  else
    super
  end
end