Volume with NULL volume_type encoded as "None"

Create a volume without specifying a volume_type. If you do a subsequent GET on that volume, the volume_type will be presented as "None" rather than null:

{
    "volumes": [
        {
            "attachments": [],
            "availability_zone": "nova",
            "created_at": "2012-10-27T23:03:41.000000",
            "display_description": null,
            "display_name": null,
            "id": "de86d99e-24fc-42d1-aa58-06c614a98f8b",
            "metadata": {},
            "size": 3,
            "snapshot_id": null,
            "status": "available",
            "volume_type": "None"
        }
    ]
}