title: convert array to CSV file on ruby
body: <p>Given array:</p>
  
  <pre><code>a = [[111,"Tue, 30 Jun 2015 23:50:55 KST +09:00"],[123,"Tue, 30 Jun 2015 23:50:55 KST +09:00"]]
  </code></pre>
  
  <p>And I want to convert this array to a new CSV file <strong>WITH NEW COLUMNS</strong> which would look like:</p>
  
  <pre><code>user_id | created_at
  --------------------
  111     | Tue, 30 Jun 2015 23:50:55 KST +09:00
  123     | Tue, 30 Jun 2015 23:50:55 KST +09:00
  </code></pre>
  
  <p>How can I do this in Ruby? 
  Thanks in advance.</p>