If we inserted "Last-Modified" in r->headers_out.headers, don't
forget to set the r->headers_out.last_modified pointer to it.

src/http/modules/ngx_http_headers_filter_module.c

diff --git a/src/http/modules/ngx_http_headers_filter_module.c b/src/http/modules/ngx_http_headers_filter_module.c
index d061fff..9389934 100644
--- a/src/http/modules/ngx_http_headers_filter_module.c
+++ b/src/http/modules/ngx_http_headers_filter_module.c
@@ -385,6 +385,8 @@
             return NGX_ERROR;
         }
 
+        *old = h;
+
     } else {
         h = *old;