In the REST DSL I turned bindingMode off like:
<post uri="/postobject" bindingMode="off">
Then it worked. I could get the response back as OK instead of "OK" with quotes.
Even with bindingMode set to auto, I would get "OK" instead of OK.
I am not sure if this is a bug or not, at least with bindingMode set to auto it should not try to convert my response which is just plain string to a json. I have set the content-type to be text/plain.