How to understand why it is not enabled for HTTP/2?

Debian 7.
Updated nginx from 1.6 to 1.10, added cherished symbols http2 Directive listen, restarted nginx. But the site continues to be given by HTTP/1.1.
The certificate is configured for a long time, in the Security panel in the DevTools, all green, no errors. Browser HTTP/2 support, of course. Nginx configuration test passes, when you reboot no errors. In the log only one warning:

2016/06/27 15:49:33 [warn] 24676#24676: duplicate MIME type "text/html" in /etc/nginx/nginx.conf:37

In the output of nginx -V is the flag --with-http_v2_module.
There are two pitfalls:

1. Nginx acts as a frontend, and proxy requests to apache.
   
2. The Directive listen is not only the port but the ip: listen 1.2.3.4:443 ssl default_server http2;.
   

Where to look, what to twist?