Fix a CalendarExport issue when an existing export has an unsupported mode

Summary:
See D16676. When an export has an unsupported mode (bad database value, out-of-date object, etc) the intent of this code is to put it into the <select /> so that you can save the form without silently changing the object.

However, it incorrectly calls array_shift() instead of array_unshift().

Test Plan:
Edited a Calendar export with an invalid mode, saw the mode appear properly in the dropdown:

Reviewers: vrana, chad

Reviewed By: vrana

Differential Revision: https://secure.phabricator.com/D17369