Unnecessary BEGIN/COMMIT happens when model save is a no-op

irb session

Loading development environment (Rails 4.1.8)
irb(main):001:0> u = User.first
  User Load (0.1ms)  SELECT  "users".* FROM "users"   ORDER BY "users"."id" ASC LIMIT 1
=> #<User id: 1, name: "Fryguy", created_at: "2014-12-05 21:18:18", updated_at: "2014-12-05 21:18:18">
irb(main):002:0> u.save!
   (0.2ms)  begin transaction
   (0.1ms)  commit transaction
=> true
irb(main):003:0> u.name = "Fryguy"
=> "Fryguy"
irb(main):004:0> u.save!
   (0.1ms)  begin transaction
   (0.1ms)  commit transaction
=> true