proxy cache + pseudo-streaming for mp4/flv

[bossfrog]

Hi guys!
In order to make the `proxy cache` and `pseudo-streaming for mp4/flv` work
together, I do a rough hacking on nginx 1.9.12 (there is my patch:
https://pastebin.mozilla.org/8869689). It works well apparently, but I
wonder that if I did it right? or I was taking a risk?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,266660,266660#msg-266660