Minor tweaks for XHPAST config

Summary: Ref T2039. The type specs are right in theory but not quite correct in practice, since we pass strings in rather than objects.

Test Plan: While tweaking phabricator/, adjusted these to get desirable results.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T2039

Differential Revision: https://secure.phabricator.com/D9071