TaxSummary doesn't include shipping tax

[martijnvdbrug]

Describe the bug
The OrderTaxSummary doesn't include the tax of shipping. The summary only holds the total per tax rate of the items in the order.

To Reproduce
Steps to reproduce the behavior:

1. Create a shippingMethod which includes tax
2. Create an order with 1 item.
3. Add the shippingMethod with tax to the order
4. Request the order.OrderTaxSummary in the GraphQL API
5. The taxSummary only holds the tax of the order items.

Expected behavior
(See result below) order.taxSummary.taxTotal is 399, but the order.total - order.totalWithTax = 516, because the order.totalWithTax includes the shipping tax aswell.

The order.taxSummary.taxTotal should be 516 (117 of shipping tax + 399 of order item tax)

Environment (please complete the following information):

• @vendure/core version: 0.18.4
• Nodejs version: v12.10.0
• Database (mysql/postgres etc): MySQL

Additional context
Result of an order with missing shipping-tax in summary

    "order": {
      "id": "12",
      "subTotal": 1901,
      "subTotalWithTax": 2300,
      "shipping": 558,
      "shippingWithTax": 675,
      "total": 2459,
      "totalWithTax": 2975,
      "taxSummary": [
        {
          "description": "Standard Tax Europe",
          "taxRate": 21,
          "taxBase": 1901,
          "taxTotal": 399
        }
      ]
    }
  }

GraphQL request for retrieving the order:

{
  order(id: "12") {
    id
    subTotal
    subTotalWithTax
    shipping
    shippingWithTax
    total
    totalWithTax
    taxSummary {
      description
      taxRate
      taxBase
      taxTotal
    }
  }
}