CVE-2016-1000109: Ignore Proxy HTTP header from fastcgi requests

Summary:
The default PHP behavior is to transform a Proxy header from requests
into the HTTP_PROXY environment variable in the PHP environment running the
request. Some scripts may use this env var to decide which HTTP proxy to
connect to, and we don't want requests to have that kind of control over
scripts' behavior.

Reviewed By: alexmalyshev, Orvid

Differential Revision: D3579221

fbshipit-source-id: 18a405fd73aee65283aea99ff995f8082b3fc3b5