You are probably having troubles with the CSRF verification that comes with rails. Check your logs and if you see the following line, then that's the reason:
WARNING: Can't verify CSRF token authenticity
A workaround can be to create a "secret/master" token to use for your tests to by pass the CSRF verification, then tell your application to accept it as a valid, for example, add the following line in your ApplicationController right after the "protect_from_forgery" line:
skip_before_filter :verify_authenticity_token, :if =>lambda{ params[:authenticity_token].present? && params[:authenticity_token] == 'YOUR_SECRET_TOKEN' }
then on the httperf tests, you should send the your master token with the params: &authenticity_token=YOUR_SECRET_TOKEN