Add Spree::StockLocation#customer_returns

We have a foreign key here, but no `has_many` declaration. Because
customer returns are important, and potentially carry return items, this
needs to be present so we can present the user with a good error message
when trying to delete a stock location with existing customer returns.

Author: mamhoff

core/app/models/spree/stock_location.rb

@@ -15,6 +15,7 @@ class InvalidMovementError < StandardError; end
     has_many :stock_movements, through: :stock_items
     has_many :user_stock_locations, dependent: :delete_all
     has_many :users, through: :user_stock_locations
+    has_many :customer_returns, inverse_of: :stock_location, dependent: :restrict_with_error
 
     belongs_to :state, class_name: 'Spree::State', optional: true
     belongs_to :country, class_name: 'Spree::Country', optional: true

core/spec/models/spree/stock_location_spec.rb

@@ -4,14 +4,22 @@
 
 module Spree
   RSpec.describe StockLocation, type: :model do
-    subject { create(:stock_location_with_items, backorderable_default: true) }
+    subject(:stock_location) { create(:stock_location_with_items, backorderable_default: true) }
     let(:stock_item) { subject.stock_items.order(:id).first }
     let(:variant) { stock_item.variant }
 
     it 'creates stock_items for all variants' do
       expect(subject.stock_items.count).to eq Variant.count
     end
 
+    describe "#customer_returns" do
+      let(:customer_return) { create(:customer_return, stock_location: stock_location) }
+
+      it "works" do
+        expect(stock_location.customer_returns).to include(customer_return)
+      end
+    end
+
     context "handling stock items" do
       let!(:variant) { create(:variant) }