Improves Spree::Wombat::Response serializer to handle objects

Hey,

This fixes ResponderSerializer to add objects to the root of the json:
```
{
    "request_id": "12355",
    "summary": "Order abc124 was added",
    "products": [
        {
            "id": "abc",
            "name": "Epic awesome tiger pyjamas"
        }
    ],
    "ninjas": [
        {
            "id": 1,
            "stars": "steel"
        },
        {
            "id": 2,
            "stars": "iron"
        }
    ]
}
```
Instead of having it under an `objects` key.

Also, I think `webhook_controller` was supposed to wrap the responder with the ResponserSerializer so we can customize the serialization.

I've also included a spec to make sure future changes won't break it.

Cheers :beers: 

Bruno