Always require invoice VAT rate

Closes #1031

test/models/invoice/vat_rate_calculator_test.rb

@@ -0,0 +1,32 @@
+require 'test_helper'
+
+class Invoice::VatRateCalculatorTest < ActiveSupport::TestCase
+  def test_applies_registry_vat_rate_when_registrar_is_vat_liable_locally
+    registry_vat_rate = 20
+    registry = Registry.new(vat_rate: registry_vat_rate, vat_country: Country.new(:us))
+    registrar = Registrar.new(vat_rate: 10, vat_country: Country.new(:us))
+
+    vat_calculator = Invoice::VatRateCalculator.new(registry: registry, registrar: registrar)
+
+    assert_equal registry_vat_rate, vat_calculator.calculate
+  end
+
+  def test_applies_registrar_vat_rate_when_registrar_is_vat_liable_in_foreign_country
+    registrar_vat_rate = 20
+    registry = Registry.new(vat_rate: 10, vat_country: Country.new(:gb))
+    registrar = Registrar.new(vat_rate: registrar_vat_rate, vat_country: Country.new(:us))
+
+    vat_calculator = Invoice::VatRateCalculator.new(registry: registry, registrar: registrar)
+
+    assert_equal registrar_vat_rate, vat_calculator.calculate
+  end
+
+  def test_applies_zero_vat_rate_when_registrar_is_vat_liable_in_foreign_country_and_vat_rate_is_absent
+    registry = Registry.new(vat_country: Country.new(:gb))
+    registrar = Registrar.new(vat_rate: nil, vat_country: Country.new(:us))
+
+    vat_calculator = Invoice::VatRateCalculator.new(registry: registry, registrar: registrar)
+
+    assert vat_calculator.calculate.zero?
+  end
+end