Sorter field: Merge arrays without modifying indexes

When using the sorter field with dynamic content where ID's are used as array keys there is a bug when using ``array_merge`` and ``array_unshift`` due to the fact they modify the array index.

This fixes the issues in #1102.

Example option config:
```
array(
    'id'        => 'opt-homepage-layout',
    'type'      => 'sorter',
    'title'     => 'Layout Manager Advanced',
    'subtitle'  => 'You can add multiple drop areas or columns.',
    'compiler'  => 'true',
    'data'      => array('disabled'=>'posts'),
    'args'      => array('disabled'=>array('post_type'=>'home', 'posts_per_page'=>-1)),
    'options'   =>  array(
        'enabled'   => array(),
        'disabled'  => array(),
    ),
),
```