Re-implement pre-Rails-5 versions of #implement! and #decrement! on Spree::LineItem

Rails 5 introduced some breaking changes to these built-in methods, and the new versions no longer work correctly in relation to decrementing stock with VariantOverrides.

app/models/concerns/line_item_stock_changes.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+# Rails 5 introduced some breaking changes to these built-in methods, and the new versions
+# no longer work correctly in relation to decrementing stock with LineItems / VariantOverrides.
+# The following methods re-instate the pre-Rails-5 versions, which work as expected.
+# https://apidock.com/rails/v4.2.9/ActiveRecord/Persistence/increment%21
+# https://apidock.com/rails/v4.2.9/ActiveRecord/Persistence/decrement%21
+
+module LineItemStockChanges
+  extend ActiveSupport::Concern
+
+  def increment!(attribute, by = 1)
+    increment(attribute, by).update_attribute(attribute, self[attribute])
+  end
+
+  def decrement!(attribute, by = 1)
+    decrement(attribute, by).update_attribute(attribute, self[attribute])
+  end
+end

app/models/spree/line_item.rb

@@ -6,6 +6,7 @@ require 'variant_units/variant_and_line_item_naming'
 module Spree
   class LineItem < ActiveRecord::Base
     include VariantUnits::VariantAndLineItemNaming
+    include LineItemStockChanges
     include LineItemBasedAdjustmentHandling
 
     belongs_to :order, class_name: "Spree::Order", inverse_of: :line_items