Restricted keepalive_disable safari to OS X only.
Restricted keepalive_disable safari to OS X only.

The problem doesn't affect non-Apple systems for sure, and many pretend
to be Safari now.

Prodded by Piotr Sikora.

firebase/nginx,firebase/nginx,firebase/nginx,firebase/nginx,hy0kl/nginx,hy0kl/nginx,hy0kl/nginx