R1:b9778d89ca93

Fix assign-on-close behavior for comment controller

Summary: Ref T2217. Missed this while converting this endpoint.

Test Plan: Closed an unassigned task, got assigned.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7089