Isn't it ironic that even the writers of this article don't follow these rules in the examples on that page?

There is "Mistake 3: Not Enabling Keepalive Connections to Upstream Servers", where it says:

    In the location{} block that forwards requests to an upstream group, include the following directives along with the proxy_pass directive:

    proxy_http_version 1.1;
    proxy_set_header   "Connection" "";

Then, at "Mistake 10: Not Taking Advantage of Upstream Groups", the "location" block does not include these two directives, even though the "upstream" block contains a "keepalive" directive.