Tax category not updated from Admin UI

[michaelbromley]

Describe the bug
Changing the tax category of a ProductVariant in the Admin UI and clicking "update" does not persist the new selection. No tax category id is sent to the API.

To Reproduce
Steps to reproduce the behavior:

1. Go to a product variant detail page
2. Choose a different tax category
3. Click "update"
4. Note that it is not persisted

Expected behavior
It should send the tax category ID with the mutation

Environment (please complete the following information):

• @vendure/core version: 2.0.5
• Nodejs version: any
• Database (mysql/postgres etc): any