special case for when theres only one promise, wrap args in an array

Summary: fixes T935

Test Plan: try creating a new form, immediately editing it, then adding the first field... then try adding more fields

Reviewers: rjmackay, vladimir, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T935

Differential Revision: https://phabricator.ushahidi.com/D451