Fix alias in getSelectColumns when using MODX3 classes

[sdrenth]

This fixes the alias in getSelectColumns when using MODX3 classes.

Before the query would result in:

SELECT `\Sterc\Site\Model\Logo`.`id`, `\Sterc\Site\Model\Logo`.`title`, `\Sterc\Site\Model\Logo`.`url`, `\Sterc\Site\Model\Logo`.`logo`, `\Sterc\Site\Model\Logo`.`default`, `\Sterc\Site\Model\Logo`.`active` FROM `6sn7rm_sterc_logo` AS `Logo` ORDER BY id ASC LIMIT 10 

After the fix it results in:

SELECT `Logo`.`id`, `Logo`.`title`, `Logo`.`url`, `Logo`.`logo`, `Logo`.`default`, `Logo`.`active` FROM `6sn7rm_sterc_logo` AS `Logo` ORDER BY id ASC LIMIT 10