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

We found a lot of orphaned records in our database due to this missing
dependent: :destroy.

core/app/models/spree/order.rb

@@ -104,7 +104,7 @@ def states
              foreign_key: :order_id,
              dependent: :destroy,
              inverse_of: :order
-    has_many :order_promotions, class_name: 'Spree::OrderPromotion'
+    has_many :order_promotions, class_name: 'Spree::OrderPromotion', dependent: :destroy
     has_many :promotions, through: :order_promotions
 
     # Payments

core/spec/models/spree/order_spec.rb

@@ -1818,4 +1818,18 @@ def validate(line_item)
       end
     end
   end
+
+  describe "order deletion" do
+    let(:order) { create(:order) }
+    let(:promotion) { create(:promotion) }
+
+    subject { order.destroy }
+    before do
+      order.promotions << promotion
+    end
+
+    it "deletes join table entries when deleting an order" do
+      expect { subject }.to change { Spree::OrderPromotion.count }.from(1).to(0)
+    end
+  end
 end