title: rails import with on_duplicate_key_update not working in mysql db
body: <p>I had an issue, when im using on duplicate key update in MySQL DB the SQL show on the log doesn't use on duplicate key update.</p>
  <p>Code</p>
  <pre><code>MyModel.import a, validate: false, on_duplicate_key_update: [:name, :company_id]
  </code></pre>
  <p>SQL Log</p>
  <pre><code>INSERT INTO `tbl_multiple_break` (`id`,`name`,`company_id`) VALUES (100,'test 101',1)   
  </code></pre>
  <p><a href="https://i.stack.imgur.com/xmVA7.png" rel="nofollow noreferrer">rails console</a></p>
  <p>because of that my import process always throws an error record not unique.
  '''</p>