Add dependent: :destroy to Spree::Order#friendly_promotions

This is the same change as in
solidusio#5411

friendly_promotions/app/decorators/models/solidus_friendly_promotions/order_decorator.rb

@@ -5,6 +5,7 @@ module OrderDecorator
     def self.prepended(base)
       base.has_many :friendly_order_promotions,
         class_name: "SolidusFriendlyPromotions::OrderPromotion",
+        dependent: :destroy,
         inverse_of: :order
       base.has_many :friendly_promotions, through: :friendly_order_promotions, source: :promotion
     end

friendly_promotions/spec/models/spree/order_spec.rb

@@ -19,4 +19,18 @@
       subject
     end
   end
+
+  describe "order deletion" do
+    let(:order) { create(:order) }
+    let(:promotion) { create(:friendly_promotion) }
+
+    subject { order.destroy }
+    before do
+      order.friendly_promotions << promotion
+    end
+
+    it "deletes join table entries when deleting an order" do
+      expect { subject }.to change { SolidusFriendlyPromotions::OrderPromotion.count }.from(1).to(0)
+    end
+  end
 end