title: Letter Opener Gem in Rails 4 doesn't open link in preview
body: <p>I'm using the <a href="https://github.com/ryanb/letter_opener" rel="nofollow">Letter Opener gem</a> to preview emails but if I click on a link that routes to an url in my application - nothing happens.</p>
  
  <p>If I copy the link url from the preview and paste it into the address bar the link works...</p>
  
  <p>Any idea?</p>
  
  <p><strong>config/development.rb</strong></p>
  
  <pre><code>  config.action_mailer.delivery_method = :letter_opener
    config.action_mailer.default_url_options = { host: 'localhost:3000' }
  </code></pre>
  
  <p><strong>views/app_mailer/send_mail.html.erb</strong></p>
  
  <pre><code>&lt;%= link_to "Home Page", root_url %&gt;
  </code></pre>
  
  <p><strong>The actual HTML</strong></p>
  
  <pre><code>&lt;a href="http://localhost:3000/"&gt;Home&lt;/a&gt;
  </code></pre>