Upstream: fixed $upstream_status when upstream returns 503/504.

If proxy_next_upstream includes http_503/http_504, and upstream
returns 503/504, $upstream_status converted this to 502 for any
values except the last one.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 2ea521b..c29ee97 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -4111,6 +4111,7 @@
     switch (ft_type) {
 
     case NGX_HTTP_UPSTREAM_FT_TIMEOUT:
+    case NGX_HTTP_UPSTREAM_FT_HTTP_504:
         status = NGX_HTTP_GATEWAY_TIME_OUT;
         break;
 
@@ -4118,6 +4119,10 @@
         status = NGX_HTTP_INTERNAL_SERVER_ERROR;
         break;
 
+    case NGX_HTTP_UPSTREAM_FT_HTTP_503:
+        status = NGX_HTTP_SERVICE_UNAVAILABLE;
+        break;
+
     case NGX_HTTP_UPSTREAM_FT_HTTP_403:
         status = NGX_HTTP_FORBIDDEN;
         break;