Update Query.php

Change method union: add second parameter of <code>bool</code> type <code>$all</code> to be <code>TRUE</code> if union must be via sql <code>UNION ALL</code> construction and <code>FALSE</code> if via <code>UNION</code>. Default is FALSE.

This PR must be connected with this: https://github.com/yiisoft/yii2/pull/2124