set X-Accel-Buffering=no response header for nginx

add X-Accel-Buffering=no response header for nginx, so that not need set `uwsgi_buffering off;` in nginx config.


http://stackoverflow.com/questions/13672743/eventsource-server-sent-events-through-nginx#comment25280208_13673298

http://wiki.nginx.org/X-accel#X-Accel-Buffering