Use header()’s http_response_code parameter

This allows PHP to send the status response header in a way that's suitable for webservers whether they require the [HTTP](http://tools.ietf.org/html/rfc2616#section-6.1) or [CGI](http://tools.ietf.org/html/rfc3875#section-6.3.3) method.

[Related change in Symphony core](https://github.com/symphonycms/symphony-2/commit/82341ea766cdf05b89f08d1ed1985c7124984670).

[Related discussion](https://github.com/symphonycms/symphony-2/issues/972).