Process XML collection nodes consistently

This fixes an issue where the `@attributes` element ends up as another item in the value of an array Parameter.

This is really specific so bear with me 

The `json_decode(json_encode())` XML conversion works differently when a collection wrapper node with only one child node, is not the only child of it's parent, has an attribute and appears on a single line.

eg. This XML without line breaks
```xml
<root>
<Result>
  <Items size="1">
    <Item/>
  </Items>
  <Other/>
</Result>
</root>
```

converts to something like this 
```php
'Items' => array(
  ['@attributes'] => array(
    'size' => 1
  ),
  [0] => array()
)
```

This commit just calls unset on `@attributes` when processing it. I figured testing for it with isset() would be an unnecessary overhead.