fix r2664: ..._cache_valid did not work for stale responses
diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index be1ea10..f7d1072 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -581,6 +581,7 @@
 
     } else if (rc == NGX_HTTP_CACHE_STALE) {
 
+        c->valid_sec = 0;
         u->stale_cache = 1;
         u->buffer.start = NULL;