Set Rack run_once to false

According to http://rack.rubyforge.org/doc/SPEC.html run_once should be set to false.

Otherwise, when I use dalli store, I got responses mixed between concurrent requests. I can reproduce it with this app: https://github.com/kazjote/puma_test. 

I start puma with:
```bash
bundle exec puma -t 4:8 -b tcp://0.0.0.0:9290 -e production
```

and run script https://gist.github.com/kazjote/5118020