Make ship method display on back_end work correcly by making checkout ignore ship methods configured for backoffice only

Adding both unit and feature tests as this is important enough for that

app/helpers/enterprises_helper.rb

@@ -14,7 +14,7 @@ module EnterprisesHelper
   def available_shipping_methods
     return [] if current_distributor.blank?
 
-    shipping_methods = current_distributor.shipping_methods
+    shipping_methods = current_distributor.shipping_methods.display_on_checkout.to_a
 
     applicator = OpenFoodNetwork::TagRuleApplicator.new(current_distributor, "FilterShippingMethods", current_customer.andand.tag_list)
     applicator.filter!(shipping_methods)

app/models/spree/shipping_method_decorator.rb

@@ -30,6 +30,7 @@ Spree::ShippingMethod.class_eval do
   }
 
   scope :by_name, -> { order('spree_shipping_methods.name ASC') }
+  scope :display_on_checkout, -> { where("display_on is null OR display_on = ''") }
 
   # Return the services (pickup, delivery) that different distributors provide, in the format:
   # {distributor_id => {pickup: true, delivery: false}, ...}

spec/features/consumer/shopping/checkout_spec.rb

@@ -229,6 +229,15 @@ feature "As a consumer I want to check out my cart", js: true do
         end
       end
 
+      it "filters out 'Back office only' shipping methods" do
+        expect(page).to have_content shipping_with_fee.name
+        shipping_with_fee.update_attribute :display_on, 'back_end' # Back office only
+
+        visit checkout_path
+        checkout_as_guest
+        expect(page).not_to have_content shipping_with_fee.name
+      end
+
       context "using FilterShippingMethods" do
         let(:user) { create(:user) }
         let(:customer) { create(:customer, user: user, enterprise: distributor) }

spec/helpers/enterprises_helper_spec.rb

@@ -28,6 +28,14 @@ describe EnterprisesHelper, type: :helper do
         expect(helper.available_shipping_methods).to_not include other_distributor_shipping_method
         expect(helper.available_shipping_methods).to include distributor_shipping_method
       end
+
+      it "does not return 'back office only' shipping method" do
+        backoffice_only_shipping_method = create(:shipping_method, require_ship_address: false, distributors: [distributor], display_on: 'back_end')
+
+        expect(helper.available_shipping_methods).to_not include backoffice_only_shipping_method
+        expect(helper.available_shipping_methods).to_not include other_distributor_shipping_method
+        expect(helper.available_shipping_methods).to include distributor_shipping_method
+      end
     end
 
     context "when FilterShippingMethods tag rules are in effect" do