I had a very similar problem using nginx to front rainbows/rails. Even though rails was definitely returning a JSON response, ngnix always responded with 200 OK, Content-Length: 0, and an empty body.

The strange thing was that the error was related to the size of the incoming POST. When the request body was small everything worked fine, the full response from Rails appeared. But as soon as the request was >100kb the response was always blank.

In my case, the logs were filled with errors like "readv() failed (104: Connection reset by peer) while reading upstream." That error led me to this post http://stackoverflow.com/questions/10393203/error-readv-failed-104-connection-reset-by-peer-while-reading-upstream. It turns out that if you do not read incoming POST data (in Rails, using request.body.read) this will cause the "connection reset" error which in turn results in a blank response body.