fix zero length static response, the bug was introduced in r2378

src/http/modules/ngx_http_static_module.c

diff --git a/src/http/modules/ngx_http_static_module.c b/src/http/modules/ngx_http_static_module.c
index e90d83c..9ff1f81 100644
--- a/src/http/modules/ngx_http_static_module.c
+++ b/src/http/modules/ngx_http_static_module.c
@@ -217,7 +217,7 @@
         return NGX_HTTP_INTERNAL_SERVER_ERROR;
     }
 
-    if (of.size == 0) {
+    if (r != r->main && of.size == 0) {
         return ngx_http_send_header(r);
     }