Fix bugs identified during product export

Arrays can't have duplicate keys. There's no such restriction on XML documents. 

What if we want to build an XML element that has multiple instances of the same child element? For example, what if we want to build this: 

```XML
<seller-tags>
    <seller-tag>kitchen</seller-tag>
    <seller-tag>utensils</seller-tag>
</seller-tags>

With this change, you can now do this: 

```php
'seller-tags' => array(
    array('seller-tag' => 'kitchen'),
    array('seller-tag' => 'utensils')
);
```