title: ActiveRecord Migration - datetime to timestamp
body: <p>I am trying to migrate some datetime columns in MySQL to timestamp type. In ActiveRecord migration I have:</p>
  
  <pre><code>change_column(:oe_tags, :created_on, :timestamp)
  </code></pre>
  
  <p>however the sql it generates is</p>
  
  <pre><code>ALTER TABLE `oe_tags` CHANGE `created_on` `created_on` datetime DEFAULT NULL
  </code></pre>
  
  <p>which is obviously NOT a timestamp type, but rather datetime (which it already is). How do I get an alteration to <code>timestamp</code>? According to the documentation :timestamp is a valid symbol. </p>
  
  <p>Edit: 
  Perhaps I did not make myself clear (apologies). I would like to know the proper change_column specification in ActiveRecord migration in Rails. I already KNOW the proper SQL I expect. How to achieve it via an ActiveRecord Migration is the question. </p>