better not delete all to avoid data loss

pr #2324

```
$result = Article::find()->query(["field" => ["title" => "yii"]])->all();
````

- DbMessageSource now performs UNION ALL.
- Fixed message merging for php and po.
- Updated tests.

Comment test because it proceed an errors when tries to run at mssql and other DBMS.

Add test for UNION/UNION ALL function.
It could be useful to use "phpunit --group=db --filter testBuildUnion" command for run it.

fixes #1585