Change app config to keep using the same v1 shipping method calculators and not the ones spree v2 introduces, these are copy pasted versions that work with packages

See this commit for more details: 18e5b98f5c (diff-b0846898827183f530c113ad7b83b8ea)

Also:
- remove shipping method restriction on calculators to inherit from Spree::Shipping::ShippingCalculator so that OFN customized calculators keep working
- add shipping method serializer spec to test serialization of all shipping methods configured

app/models/spree/shipping_method_decorator.rb

@@ -42,6 +42,14 @@ Spree::ShippingMethod.class_eval do
     ]
   end
 
+  # This method is overriden so that we can remove the restriction added in Spree
+  #   Spree restricts shipping method calculators to the ones that inherit from Spree::Shipping::ShippingCalculator
+  #   Spree::Shipping::ShippingCalculator makes sure that calculators are able to handle packages and not orders as input
+  #   This is not necessary in OFN because calculators in OFN are already customized to work with different types of input
+  def self.calculators
+    spree_calculators.send model_name_without_spree_namespace
+  end
+
   def has_distributor?(distributor)
     self.distributors.include?(distributor)
   end

config/application.rb

@@ -55,7 +55,15 @@ module Openfoodnetwork
 
     # Register Spree calculators
     initializer 'spree.register.calculators' do |app|
-      app.config.spree.calculators.shipping_methods << Calculator::Weight
+      app.config.spree.calculators.shipping_methods = [
+        Spree::Calculator::FlatPercentItemTotal,
+        Spree::Calculator::FlatRate,
+        Spree::Calculator::FlexiRate,
+        Spree::Calculator::PerItem,
+        Spree::Calculator::PriceSack,
+        Calculator::Weight
+      ]
+
       app.config.spree.calculators.add_class('enterprise_fees')
       config.spree.calculators.enterprise_fees = [
         Calculator::FlatPercentPerItem,

spec/serializers/api/shipping_method_serializer_spec.rb

@@ -0,0 +1,21 @@
+require 'spec_helper'
+
+describe Api::ShippingMethodSerializer do
+  let(:shipping_method) { create(:shipping_method) }
+
+  it "serializes a test shipping_method" do
+    serializer = Api::ShippingMethodSerializer.new shipping_method
+
+    expect(serializer.to_json).to match(shipping_method.name)
+  end
+
+  it "can serialize all configured shipping method calculators" do
+    Rails.application.config.spree.calculators.shipping_methods.each do |calculator|
+      shipping_method.calculator = calculator.new
+      serializer = Api::ShippingMethodSerializer.new shipping_method
+      allow(serializer).to receive(:options).and_return(current_order: create(:order))
+
+      expect(serializer.price).to eq(0.0)
+    end
+  end
+end