Fix HTTP 400 from notification server for JSON subscription objects

Summary: Fixes T5651. Sometime we'll send an object to the notification server for `subscribers`, which it will choke on. Use `array_values()` to make sure we're sending an array.

Test Plan: With `(object)` instead, got a consistent error ("no .filter method on object"). With `array_values()`, no error.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5651

Differential Revision: https://secure.phabricator.com/D9963