title: Use Rails’ form_for but set custom classes, attributes on <form> element?
body: <p><code>form_for</code> seems to ignore any 'extra' attributes like a <code>data-foo</code> attribute or <code>class</code> passed as <code>options</code> in its second argument.</p>
  
  <pre><code>= form_for @user, {:url =&gt; 'foo', :class =&gt; 'x', 'data-bar' =&gt; 'baz' } do |f|
    # ...
  </code></pre>
  
  <p>The output is a <code>&lt;form&gt;</code> tag with no <code>x</code> class or <code>data-bar</code> attribute.</p>
  
  <p>What’s the fix?</p>
  
  <p>Or, how can I grab a <code>FormBuilder</code> instance without using <code>form_for</code>?</p>