Don't force users to match existing config

If an object is in the registry already, and no bespoke config has been
specified - don't consider that an exceptional case.

The scenario which brought me here was having one helper like this:

```
<?php
namespace App\View\Helper;

use Cake\Core\Configure;
use Cake\View\View;
use Cake\View\Helper\UrlHelper as CakeUrlHelper;
use Cake\Routing\Router;

/**
 * Url helper
 */
class UrlHelper extends CakeUrlHelper {

    public function __construct(View $View, array $config = []) {
        $this->config('assetHost', Configure::read('static.host'));

        parent::__construct($View, $config);
    } 
```

And another like this:

```
<?php
namespace App\View\Helper;

use Cake\View\Helper;
use Cake\View\View;

/**
 * Breadcrumbs helper
 */
class BreadcrumbsHelper extends Helper {

    public $helpers = [ 
        'Url'
    ];  
```

And being confronted with:

> An Internal Error Has Occurred
Error: The "Url" alias has already been loaded with the following config: array ( 'assetHost' => '//static.example.dev', ) which differs from array ( )