[Rails5] Fix optimistic lock value

Update the monkey-patch to make it work in Rails 5.

changelogs/unreleased/blackst0ne-rails5-fix-optimistic-lock-values.yml

+---
+title: "[Rails5] Fix optimistic lock value"
+merge_request: 19878
+author: "@blackst0ne"
+type: fixed

config/initializers/active_record_locking.rb

 # rubocop:disable Lint/RescueException
 
-# Remove this entire initializer when we are at rails 5.0.
-# This file fixes the bug (see below) which has been fixed in the upstream.
-unless Gitlab.rails5?
-  # This patch fixes https://github.com/rails/rails/issues/26024
-  # TODO: Remove it when it's no longer necessary
-
-  module ActiveRecord
+# Remove this monkey-patch when all lock_version values are converted from NULLs to zeros.
+# See https://gitlab.com/gitlab-org/gitlab-ce/issues/25228
+module ActiveRecord
   module Locking
     module Optimistic
       # We overwrite this method because we don't want to have default value
@@ -68,6 +64,17 @@ unless Gitlab.rails5?
 
     # This is patched because we want `lock_version` default to `NULL`
     # rather than `0`
+    if Gitlab.rails5?
+      class LockingType
+        def deserialize(value)
+          super
+        end
+
+        def serialize(value)
+          super
+        end
+      end
+    else
       class LockingType < SimpleDelegator
         def type_cast_from_database(value)
           super