title: ActiveRecord migration :unique => true being ignored?
body: <p>I am creating a table and specifying:</p>
  
  <pre><code>t.integer   :random_id, :unique =&gt; true
  </code></pre>
  
  <p>It doesn't add a unique index to random_id. But if I do:</p>
  
  <pre><code>ActiveRecord::Migration.add_index :test, :random_id, :unique =&gt; true
  </code></pre>
  
  <p>Then it works. What's wrong with the first way?
  THanks</p>