Rails更新内容，并不变化 updated_at

正常我们调用 model.save 保存的时候会更新 timestamp，但有些时候我们不希望更新。

>> user.updated_at
=> Wed, 16 Mar 2016 09:15:30 UTC +00:00

>> user.name = "Dan"
>> user.save(touch: false)
  UPDATE "users" SET "name" = ? WHERE "users"."id" = ?  [["name", "Dan"], ["id", 12]]
=> true

>> user.updated_at
=> Wed, 16 Mar 2016 09:15:30 UTC +00:00