allow the upstream modules to send a header only

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 00fa457..f9d2482 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1356,7 +1356,7 @@
 
     rc = ngx_http_send_header(r);
 
-    if (rc == NGX_ERROR || rc > NGX_OK || r->post_action) {
+    if (rc == NGX_ERROR || rc > NGX_OK || r->post_action || r->header_only) {
         ngx_http_upstream_finalize_request(r, u, rc);
         return;
     }