Remove trailing zeroes in tax amount

NumberToPercentageConverter defaults to a precision of 3, leading to
displays of VAT amount like '21.000%' on the cart's label, which doesn't
look good. Using `precision: nil` will keep the default precision of the
number.

backend/spec/features/admin/orders/adjustments_spec.rb

@@ -37,7 +37,7 @@
     context "admin managing adjustments" do
       it "should display the correct values for existing order adjustments" do
         within first('table tr', text: 'Tax') do
-          expect(column_text(2)).to match(/TaxCategory - \d+ 20\.000%/)
+          expect(column_text(2)).to match(/TaxCategory - \d+ 20\.0%/)
           expect(column_text(3)).to eq("$2.00")
         end
       end

core/app/models/spree/tax_rate.rb

@@ -118,7 +118,8 @@ def adjustment_label(amount)
     def amount_for_adjustment_label
       ActiveSupport::NumberHelper::NumberToPercentageConverter.convert(
         amount * 100,
-        locale: I18n.locale
+        locale: I18n.locale,
+        precision: nil
       )
     end
 

core/spec/models/spree/tax_calculator/default_spec.rb

@@ -46,7 +46,7 @@
       expect(line_item_tax.amount).to eq 1
       expect(line_item_tax.included_in_price).to be false
       expect(line_item_tax.tax_rate).to eq book_tax_rate
-      expect(line_item_tax.label).to eq "New York Sales Tax 5.000%"
+      expect(line_item_tax.label).to eq "New York Sales Tax 5.0%"
     end
 
     it "has tax information for the shipments" do