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.phabricator.com/D18975