mailer previews, make sure labels and values line up.

While this was true before when every `dd` had a value,
this patch makes sure that everything keeps lining up even
when the `dd` node is blank.

actionmailer/CHANGELOG.md

+*   Make sure labels and values line up in mailer previews.
+
+    *Yves Senn*
+
 *   Add `assert_enqueued_emails` and `assert_no_enqueued_emails`.
 
     Example:

railties/lib/rails/templates/rails/mailers/email.html.erb

@@ -39,6 +39,10 @@
     padding: 1px;
   }
 
+  dd:empty:before {
+    content: "\00a0"; //  
+  }
+
   iframe {
     border: 0;
     width: 100%;