Support DELETE http method in libphutil

Summary: Ref T2852. Asana uses this for DELETEing stuff. This array was just to catch typos, I'm pretty sure (I think there was a `setMethod('PSOT')` somewhere once), and the method works correctly.

Test Plan: Deleted asana stuff via API.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6309