Simplify Order#total_tax to cover all taxes from all sources

app/models/spree/order.rb

@@ -662,9 +662,7 @@ module Spree
     end
 
     def total_tax
-      adjustments.sum(:included_tax) +
-        shipment_adjustments.sum(:included_tax) +
-        line_item_adjustments.tax.sum(:amount)
+      additional_tax_total + included_tax_total
     end
 
     def has_taxes_included

spec/models/spree/order_spec.rb

@@ -699,7 +699,7 @@ describe Spree::Order do
         amount: 123,
         included_tax: 2
       )
-      order.reload
+      order.update!
     end
 
     it "returns a sum of all tax on the order" do