title: ActiveAdmin (Rails) empty class attribute for custom index table rows
body: <p>I am using ActiveAdmin to render an index table for a model.</p>
  
  <p>The config looks like this:</p>
  
  <pre><code>ActiveAdmin.register User do
    index do
      selectable_column
      column :username
      column "Email" do |u|
        raw "&lt;span title='#{u.email}'&gt;#{truncate(u.email, length: 14)}&lt;/span&gt;"
      end
    end
  end
  </code></pre>
  
  <p>The rendered HTML for the "email" TD looks like this:</p>
  
  <pre><code>  &lt;td class=""&gt;&lt;span title="useremail@example.com"&gt;useremail...&lt;/span&gt;&lt;/td&gt;
  </code></pre>
  
  <p>I have tried passing class: 'foo' before the block and a few things like that but none have worked.</p>
  
  <p>All of the columns that are not being created with the block syntax are getting a class attribute equal to the field name.</p>
  
  <p>How do I set the class for the TD when using a block to render the column?</p>