title: rails yaml fixture - specify NULL value
body: <p>I have a fixture with values like this:</p>
  
  <pre><code>product_four:
       id: 4
       application_id: 1
       title: "oldtitle"
       deleted_at: ~
  </code></pre>
  
  <p>Setting up a postgresql database for testing.
  But I can't figure out how to set the deleted_at field to NULL rather than [empty].
  I've tried:</p>
  
  <pre><code>deleted_at: :null
  deleted_at: &lt;%= nil %&gt;
  deleted_at: ~
  deleted_at: NULL
  </code></pre>
  
  <p>And probably a couple more, without luck.
  Clues for the clueless?</p>