Prevent reverse proxy from sending range headers to source server

[FSC]

No idea how I missed that. Thank you very much! 

Also for people that might find this thread later:

Adding the parameter "updating" to the directive "proxy_cache_use_stale" is
important as well if you want to minimize the traffic the origin server
generates.

proxy_cache_use_stale updating;
proxy_set_header Range "";

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,264362,264370#msg-264370