title: skipping after_update callback
body: <p>I have a model that uses <code>after_update</code> to log changes. There is a case where I would like to make a change to the model without activating this logging mechanism. Is there a way to pass in a parameter to after_update, or skip it all together?</p>
  
  <p>I would like a nice solution to this, and am willing to remove after_update if there is a better way to go about it.</p>