Export headers for relationships are in "machine name" format

Actually, it's the last relationship mapped for export that is in the "machine name" format. (By machine name I mean it's of the form id_a_b-organization_name vs Board Director of-organization_name, for example.)

Moving the call to self::manipulateHeaderRows($headerRows, $contactRelationshipTypes); - in CRM/Export/BAO/Export.php fixed this.