Fix compatibility with nginx 1.23.0+

This copies code contributed to ngx_headers_more by @zhuizhuhaomeng
in openresty/headers-more-nginx-module@d502e41

I omitted the PCRE2-related build flag added in that same commit because
I do not understand the reason for that.

This copies code contributed to ngx_headers_more by @hnakamur in
openresty/headers-more-nginx-module@91eb0db

This addresses #44