merge r3406:

fix negative time in "Cache-Control" if "expires modified" sets time in the past
diff --git a/src/http/modules/ngx_http_headers_filter_module.c b/src/http/modules/ngx_http_headers_filter_module.c
index 3b5ebd6..fb6c005 100644
--- a/src/http/modules/ngx_http_headers_filter_module.c
+++ b/src/http/modules/ngx_http_headers_filter_module.c
@@ -291,7 +291,7 @@
 
     ngx_http_time(expires->value.data, expires_time);
 
-    if (conf->expires_time < 0) {
+    if (conf->expires_time < 0 || max_age < 0) {
         cc->value.len = sizeof("no-cache") - 1;
         cc->value.data = (u_char *) "no-cache";