Don't wrap array items in root element

When serializing an array with a root element, the root option is passed down to the items. If an item's serializer supports this option, it will be again wrapped in the root element. Instead of
```
{ "root": [{ "item_id" : 1 }] }
```
we get 
```
{ "root": [{ "root" : { "item_id" : 1 } }] }
```
This is fixed by omitting the root option when we instantiate the item's serializer.