nginx: Fix gixy test http_splitting

Fixes test error:
```
>> Problem: [http_splitting] Possible HTTP-Splitting vulnerability.
Description: Using variables that can contain "\n" or "\r" may lead to http injection.
```

Summary: `$uri` should never be used in `return` statements.
See: https://github.com/yandex/gixy/blob/master/docs/en/plugins/httpsplitting.md

In this case, `$uri` always equals `/`, so just replace it.

production/nginx/server-common.conf

@@ -49,7 +49,7 @@ add_header Vary Cookie;
 # cache redirect for 10 minutes
 location = / {
 	if ($lang != '') {
-		return 302 $scheme://$host/$lang$uri;
+		return 302 $scheme://$host/$lang/;
 	}
 	try_files /en-US/index.html =404;
 	expires 10m;