Why isn't uWSGI respecting the "--http-keepalive" flag?

I was finally able to get keepalive working by switching from --http-socket to simply --http. According to uWSGI docs:

If your web server does not support the uwsgi protocol but is able to speak to upstream HTTP proxies, or if you are using a service like Webfaction or Heroku to host your application, you can use http-socket. If you plan to expose your app to the world with uWSGI only, use the http option instead, as the router/proxy/load-balancer will then be your shield.

In my particular case, it was also necessary to load the http plugin.