title: extract_subdomains returns an empty array in rails 4.2
body: <p>In previous versions of rails the extract_subdomains was working as intended but in version >4.2 it returns an empty array.</p>
  
  <pre><code>ActionDispatch::Http::URL.extract_subdomains("aalfaa.bbetaa.com", 3)
  =&gt; []
  </code></pre>