HTTP: change default response code when rate-limiting requests.

Revert default response code back to "503 Service Unavailable", because
the local change to "429 Too Many Requests" was never accepted upstream,
so it's an unnecessary incompatibility with the vanilla NGINX.

The old behavior can be restored using "limit_req_status" directive.

This reverts commit d4e033db526b0b5b0c75fd7d96e77a21f51667ad.

Change-Id: Ie0a39d63e3f354540fe71683c8373e2b4f002854
Signed-off-by: Piotr Sikora <piotrsikora@google.com>
Reviewed-on: https://nginx-review.googlesource.com/c/3520
diff --git a/src/http/modules/ngx_http_limit_req_module.c b/src/http/modules/ngx_http_limit_req_module.c
index 6648c4d..63ec2de 100644
--- a/src/http/modules/ngx_http_limit_req_module.c
+++ b/src/http/modules/ngx_http_limit_req_module.c
@@ -733,7 +733,7 @@
                                 NGX_LOG_INFO : conf->limit_log_level + 1;
 
     ngx_conf_merge_uint_value(conf->status_code, prev->status_code,
-                              NGX_HTTP_TOO_MANY_REQUESTS);
+                              NGX_HTTP_SERVICE_UNAVAILABLE);
 
     return NGX_CONF_OK;
 }