Eliminate extra queries when using Optimistic Locking with Partial Updates

If you use optimistic locking with partial updates, you get extra queries where they aren't needed.

Example (save a person without updates):

Person.first
Person.save!

Generates:

  Person Update with optimistic locking (0.000400)    UPDATE `people`
 SET `lock_version` = 14
 WHERE id = 339
 AND `lock_version` = 13

This patch duplicates the functionality of ActiveRecord::Base#update, where the update isn't run if there are no fields in the update.

Therefore, when you use locking with partial updates, you won't have extra queries floating around.