Fix an issue where PHP puts the content type in CONTENT_TYPE instead of HTTP_CONTENT_TYPE

Summary: Fixes T4084. See that task for discussion.

Test Plan: Did `git clone`. My setup doesn't precisely reproduce the original issue, but hopefully @enko can confirm this is a fix.

Reviewers: btrahan, enko

Reviewed By: enko

CC: enko, aran

Maniphest Tasks: T4084

Differential Revision: https://secure.phabricator.com/D7561