Update Model.php

Change\Fix connection behavior of model relations.

##### Examples
_Application configs:_
`database.php` 
```php
return [
    'default' => 'conn1',

    'connections' => [
        'conn1' => [ ****** ], 
        'conn2' => [ ****** ]
    ]
];
```
`models/`
```php
class Parent extends Eloquent
{
    public function children()
    {
        return $this->belongsTo('Child', 'parent_id', 'id');
    }
}
```
_Before:_
```php
Parent::with('children')->get();
// model Parent $connection = 'conn1';
// model Child $connection = 'conn1';


Parent::on('conn2')->with('children')->get();
// model Parent $connection = 'conn2';
// model Child $connection = 'conn1'; (default connection)
```

_After fix:_
```php
Parent::with('children')->get();
// model Parent $connection = 'conn1';
// model Child $connection = 'conn1';


Parent::on('conn2')->with('children')->get();
// model Parent $connection = 'conn2';
// model Child $connection = 'conn2';  (connection of parent model)
```

Sorry for the lack of unit tests, never used Mocks, only phpunit asserts =(