title: Mongoid use only created_at timestamp
body: <p>Is there anyway to set only the created_at timestamp for read-only documents?</p>
  
  <p>I currently have the following message class</p>
  
  <pre><code>class Message
    include Mongoid::Document
    include Mongoid::Timestamps
  
    field :text,      type: String
  
    belongs_to :user, foreign_key: :user_id
    embedded_in :conversation
  end
  </code></pre>
  
  <p>It works ok, but for every message I'm wasting space with the updated_at field, which will always be the same as created_at</p>