Sort order of search not preserved in Export

Follow-Up of CRM-15301

Sorting of exported contributions, memberships or contacts is not preserved from search in export.

Steps to reproduce:

• Find Contribution
• Change sort order
• Select all contributions
• Select 'export contributions' from dropdown
• sorting in csv file is not identical to search

Problem is in CRM/Export/BAO/Export.php, line 721

if (!empty($returnProperties[$field])) {
        //CRM-15301
        $queryString .= " ORDER BY $order";     
}

The if construct never becomes true and therefore the order by-clause is not appended.