bugfix: config: avoided an error with NGINX 1.17.0 and above.
bugfix: config: avoided an error with NGINX 1.17.0 and above.

The postpone filter is unconditionally enabled with NGINX 1.17.0 and
above.

See:

https://github.com/nginx/nginx/commit/a39380a41e7d7ceeda2c0526c5df474f158c6a53
openresty/echo-nginx-module,openresty/echo-nginx-module,openresty/echo-nginx-module