Add missing inverse_of for shipping_rate and taxes

Determining the shipping_rate_tax's currency delegates all the way back
to the order, which was causing unnecessary selects of the
shipping_rate, shipment, and order because the inverse_of couldn't be
determined.

core/app/models/spree/shipment.rb

@@ -7,7 +7,7 @@ class Shipment < Spree::Base
 
     has_many :adjustments, as: :adjustable, inverse_of: :adjustable, dependent: :delete_all
     has_many :inventory_units, dependent: :destroy, inverse_of: :shipment
-    has_many :shipping_rates, -> { order(:cost) }, dependent: :destroy
+    has_many :shipping_rates, -> { order(:cost) }, dependent: :destroy, inverse_of: :shipment
     has_many :shipping_methods, through: :shipping_rates
     has_many :state_changes, as: :stateful
     has_many :cartons, -> { uniq }, through: :inventory_units

core/app/models/spree/shipping_rate.rb

@@ -9,6 +9,7 @@ class ShippingRate < Spree::Base
     has_many :taxes,
              class_name: "Spree::ShippingRateTax",
              foreign_key: "shipping_rate_id",
+             inverse_of: :shipping_rate,
              dependent: :destroy
 
     delegate :order, :currency, to: :shipment