R1:c9df8f77c8b5

Fix transcription of single-value bulk edit fields ("Assign to")

Summary: See PHI333. Some of the cleanup at the tail end of the bulk edit changes made "Assign To" stop working properly, since we don't strip the `array(...)` off the `array(PHID)` value we receive.

Test Plan:
- Used bulk editor to assign and unassign tasks (single value datasource).
- Used bulk editor to change projects (multi-value datasource).

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.