Add eager loading to shipping methods in Estimator

Thanks to Package.shipping_methods now returning an AR::Relation, we are
able to eager load the associated records required for estimating
shipping rates. This should reduce a bunch of per-shipping method
queries to only run once.

core/app/models/spree/stock/estimator.rb

@@ -55,7 +55,10 @@ def calculate_shipping_rates(package)
       end
 
       def shipping_methods(package)
-        package.shipping_methods.select do |ship_method|
+        package.shipping_methods
+          .includes(:calculator, zones: :zone_members, tax_category: :tax_rates)
+          .to_a
+          .select do |ship_method|
           calculator = ship_method.calculator
           ship_method.include?(order.ship_address) &&
             calculator.available?(package) &&

core/spec/models/spree/stock/estimator_spec.rb

@@ -18,7 +18,7 @@ module Stock
           allow_any_instance_of(ShippingMethod).to receive_message_chain(:calculator, :preferences).and_return({ currency: currency })
           allow_any_instance_of(ShippingMethod).to receive_message_chain(:calculator, :marked_for_destruction?)
 
-          allow(package).to receive_messages(shipping_methods: [shipping_method])
+          allow(package).to receive_message_chain(:shipping_methods, includes: [shipping_method])
         end
 
         let(:currency) { "USD" }