Use array_replace_recursive instead of array_merge

Use ```array_replace_recursive``` instead of ```array_merge``` for ```$config```

e.g.
I had this in with the config to set the ```ActiveField``` class for forms.
```
Yii::$container->set('yii\bootstrap\ActiveForm', [
    'fieldConfig' => [
        'class' => 'common\components\ActiveField',
    ],
]);
```

When using the form I wanted to change some css classes.
```
<?php $form = ActiveForm::begin([
    'layout' => 'horizontal',
    'fieldConfig' => [
        'horizontalCssClasses' => [
            'label' => 'col-sm-2',
            'wrapper' => 'col-sm-8',
            'hint' => 'col-sm-2'
        ]
    ]
]) ?>
```
This would overwrite the initial config falling back to the default ```ActiveField``` class

If it's OK I will update the changelog.