title: update_attribute/s() is calling callback for save password
body: <p>I'm trying to update single attribute of a user model from a admin controller (not users controller). </p>
  
  <p>While doing this I tried <code>update_attribute()</code> but it was changing the users password also. </p>
  
  <p>I think the password is changing because I have before_save method on user model which hashes the password. </p>
  
  <p><code>update_attributes()</code> is not working because it is checking the validations for password which is <code>presence=&gt;true</code></p>
  
  <p>Is there any way to achieve this? </p>