nginx: upstream response is buffered to a temporary file

From `/data/logs/nginx.log`:

```
2015/09/07 01:51:34 [warn] 1679#0: *6850 an upstream response is buffered to a
temporary file /var/cache/nginx/proxy_temp/2/02/0000000022 while reading upstream,
client: 144.76.15.235, server: my-stamps.ru, request: "GET /image/92 HTTP/1.1",
upstream: "http://127.0.0.1:8080/image/92", host: "my-stamps.ru"
```
