fixed proxy chunked response security problem

http://mailman.nginx.org/pipermail/nginx-announce/2013/000114.html

A security problem related to CVE-2013-2028 was identified,
affecting some previous nginx versions if proxy_pass to
untrusted upstream HTTP servers is used.

The problem may lead to a denial of service or a disclosure of a
worker process memory on a specially crafted response from an
upstream proxied server.