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 When an export has an unsupported mode (bad database value, out-of-date object, etc) the intent of this code is to put it into the `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:

{F2957321}

Reviewers: vrana, chad

Reviewed By: vrana

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

src/applications/calendar/editor/PhabricatorCalendarExportEditEngine.php

@@ -70,7 +70,7 @@ final class PhabricatorCalendarExportEditEngine
 
     $current_mode = $object->getPolicyMode();
     if (empty($export_modes[$current_mode])) {
-      array_shift($export_modes, $current_mode);
+      array_unshift($export_modes, $current_mode);
     }
 
     $mode_options = array();