Fixes #2336: PostgreSQL: CDbCommandBuilder uses `NULL` instead of `DEFAULT` as default value for the primary keys of serial type.
Removed debugging group declaration.
Merge branch 'master' of github.com:yiisoft/yii into fixes-2336

Conflicts:
	CHANGELOG
	tests/framework/db/ar/CActiveRecord2Test.php
Changelog adjusted.
Merge pull request #2349 from resurtm/fixes-2336

Fixes #2336: PostgreSQL: CDbCommandBuilder uses `NULL` instead of `DEFAULT` as default value for the primary keys of serial type.