Support item_id normalized field on the Authorize.Net gateway

Spreedly now supports the order_data.line_items.item_id normalized field on the Authorize.Net gateway. When you send line items to Authorize.Net, each line item requires item_id, item_name, description, quantity, and unit_cost. Spreedly shortens item_id and item_name to 31 characters and description to 255 characters. The same fields (item_id, name, description, quantity, and unit_price) are required in the line_items gateway specific field. For more information, see the normalized fields guide and the gateway guide.