title: Initialize virtual attributes
body: <p>I have an <code>IncomingEmail</code> model with an <code>attachments</code> virtual attribute:</p>
  
  <pre><code>class IncomingEmail &lt; ActiveRecord::Base  
    attr_accessor :attachments
  end
  </code></pre>
  
  <p>I want the <code>attachments</code> virtual attribute to be initialized to <code>[]</code> rather than <code>nil</code> so that I can do:</p>
  
  <pre><code>&gt;&gt; i = IncomingEmail.new
  =&gt; #&lt;IncomingEmail id: nil,...)
  &gt;&gt; i.attachments &lt;&lt; "whatever"
  </code></pre>
  
  <p>Without first setting <code>i.attachments</code> to <code>[]</code> (put another way, I want this virtual attribute to default to an empty array rather than <code>nil</code>)</p>