Turning off cookies for RESTful service connected to Mule

I'm trying to get Rails 2.1 to talk to Mule 2.0. Mule is complaining
"Invalid cookie state: domain not specified". I think I can fix this
by telling Rails not to use cookies (I don't need any session
information). But I don't know how to turn off cookies. I've tried
session :disabled => true and session :off, but these don't fix the
problem.

More details:
I have a Metrics controller and model created using scaffolding.
I have used map.resources :metrics.
Mule is doing a post to /metrics.xml using Mule's http:rest-service-
component.
(I'm not currently using the data returned, but may in the future)

Now, I know the problem might actually be on the Java side in
httpclient, but I can't find any clear explanation of whether
disabling sessions will turn off all cookies (at least, all cookies
that would be set by scaffolding generated code). How do I turn off
cookies?

Thanks,
Kevin Peterson