Remove tax_rates shortcut from adjustment

Becoming less dependent on Spree, using our own namespace, keeping
decorators small.

app/helpers/checkout_helper.rb

@@ -54,7 +54,7 @@ module CheckoutHelper
   end
 
   def display_adjustment_tax_rates(adjustment)
-    tax_rates = adjustment.tax_rates
+    tax_rates = TaxRateFinder.tax_rates_of(adjustment)
     tax_rates.map { |tr| number_to_percentage(tr.amount * 100, :precision => 1) }.join(", ")
   end
 

app/models/spree/adjustment_decorator.rb

@@ -40,11 +40,6 @@ module Spree
       included_tax > 0
     end
 
-    # @return [Array<Spree::TaxRate>]
-    def tax_rates
-      TaxRateFinder.new.tax_rates(originator, source, amount, included_tax)
-    end
-
     def self.without_callbacks
       skip_callback :save, :after, :update_adjustable
       skip_callback :destroy, :after, :update_adjustable

app/models/spree/order_decorator.rb

@@ -293,7 +293,7 @@ Spree::Order.class_eval do
 
   def tax_adjustment_totals
     tax_adjustments.each_with_object(Hash.new) do |adjustment, hash|
-      tax_rates = adjustment.tax_rates
+      tax_rates = TaxRateFinder.tax_rates_of(adjustment)
       tax_rates_hash = Hash[tax_rates.collect do |tax_rate|
         tax_amount = tax_rates.one? ? adjustment.included_tax : tax_rate.compute_tax(adjustment.amount)
         [tax_rate, tax_amount]

app/services/tax_rate_finder.rb

@@ -2,6 +2,16 @@
 # For example a packaging fee may contain VAT. This service finds the VAT rate
 # for the tax included in the packaging fee.
 class TaxRateFinder
+  # @return [Array<Spree::TaxRate>]
+  def self.tax_rates_of(adjustment)
+    new.tax_rates(
+      adjustment.originator,
+      adjustment.source,
+      adjustment.amount,
+      adjustment.included_tax
+    )
+  end
+
   # @return [Array<Spree::TaxRate>]
   def tax_rates(originator, source, amount, included_tax)
     find_associated_tax_rate(originator, source) ||

lib/open_food_network/enterprise_fee_applicator.rb

@@ -32,7 +32,7 @@ module OpenFoodNetwork
     end
 
     def adjustment_tax(adjustable, adjustment)
-      tax_rates = adjustment.tax_rates
+      tax_rates = TaxRateFinder.tax_rates_of(adjustment)
 
       tax_rates.select(&:included_in_price).sum do |rate|
         rate.compute_tax adjustment.amount