title: Simple_form adding class to form
body: <p>I am using simple_form in my Rails application, I tried to add <code>form-horizontal</code> class to my form.</p>
  
  <pre><code>&lt;form accept-charset="UTF-8" action="/account/orders" class="simple_form new_order" data-validate="true" enctype="multipart/form-data" id="new_order" method="post" novalidate="novalidate"&gt;
  </code></pre>
  
  <p>When I use <code>html: { class: "form-horizontal" }</code> it change <code>class="simple_form new_order"</code> to <code>class="simple_form form-horizontal"</code>.</p>
  
  <p>What should I do to keep <code>new_order</code> class?</p>