Fixes bug introduced by D6251

Summary: Casting the defaul custom field storage to an int, need to stop that!

Test Plan:
Use some custom fields, make sure they work as expected.
Fixes T3444

Reviewers: epriestley, mbishopim3

Reviewed By: mbishopim3

CC: aran, Korvin, mbishopim3

Maniphest Tasks: T3444

Differential Revision: https://secure.phabricator.com/D6282