Let PhabricatorSearchCheckboxesField survive saved query data with mismatched types

Summary:
Fixes T12851.

This should fix the error I'm seeing, which is:

• Argument 1 passed to array_fuse() must be of the type array, boolean given

There may be a better way to patch this up than overriding the getValue() method,
however.

Test Plan:

• Changed the default "Tags" filter to specify true instead of array('self'), then viewed that filter in the UI.
• Before patch: fatal.
• After patch: page loads. Note that true is not interpreted as array('self'), but the page isn't broken, which is a big improvement.

Reviewers: Blessed Reviewers, 20after4, chad, amckinley

Reviewed By: Blessed Reviewers, amckinley

Subscribers: Korvin

Maniphest Tasks: T12851

Differential Revision: https://secure.phabricator.com/D18132