Magento 502 bad gateway nginx error (and how can you fix it)?

in nginx logs it is showing

upstream sent too big header while reading response header from upstream,

then solved this issue by modifying this file

vi /usr/local/etc/nginx/nginx.conf

add this line of codes inside http{}

fastcgi_buffers 16 16k;
fastcgi_buffer_size 32k;
proxy_buffer_size 128k;
proxy_buffers 4 256k;
proxy_busy_buffers_size 256k;