Change migration to avoid tiggering callback and use private send:

* Skip :after callbacks on StockLocation and StockItem when creating
intitial instances. Regression due to 0ba60a0 and 630afbb
* Use private send to set StockItem.count_on_hand
because :count_on_hand= is now private. Regression due to 9acf57f

[Closes solidusio#2789]

core/db/migrate/20130213191427_create_default_stock.rb

@@ -1,8 +1,12 @@
 class CreateDefaultStock < ActiveRecord::Migration
  def up
+ Spree::StockLocation.skip_callback(:create, :after, :create_stock_items)
+ Spree::StockItem.skip_callback(:save, :after, :process_backorders)
  location = Spree::StockLocation.create(name: 'default')
  Spree::Variant.all.each do |variant|
- location.stock_items.create(variant: variant, count_on_hand: variant.count_on_hand)
+ stock_item = location.stock_items.build(variant: variant)
+ stock_item.send(:count_on_hand=, variant.count_on_hand)
+ stock_item.save!
  end
 
  remove_column :spree_variants, :count_on_hand