do not send "100 Continue" for subrequests

src/http/ngx_http_core_module.c

diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index a7a5cdb..c949d49 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -862,7 +862,10 @@
         return NGX_OK;
     }
 
-    if (r->headers_in.expect && r->http_version > NGX_HTTP_VERSION_10) {
+    if (r->headers_in.expect
+        && r == r->main
+        && r->http_version > NGX_HTTP_VERSION_10)
+    {
         expect = ngx_http_core_send_continue(r);
 
         if (expect != NGX_OK) {