title: Rails form helper: Radio Button doesn't take css-class name
body: <p>I have a Rails form helper and want to add a css-class to a radio button input field.</p>
  
  <pre><code>            &lt;span class="input-nile"&gt;
                &lt;%= f.radio_button :nile_administration, class: 'input__field input__field-nile radio' %&gt;
                &lt;label class="radio-label"&gt;Nile Administration&lt;/label&gt;
              &lt;/span&gt;
  </code></pre>
  
  <p>The output is</p>
  
  <pre><code>           &lt;span class="input-nile"&gt;
                &lt;input type="radio" value="{:class=&gt;&amp;quot;input__field input__field-nile radio&amp;quot;}" name="user[nile_administration]" id="user_nile_administration_classinput__field_input__field-nile_radio"&gt;
                &lt;label class="radio-label"&gt;Nile Administration&lt;/label&gt;
              &lt;/span&gt;
  </code></pre>
  
  <p>I want to achieve that the input tag of type radio get the css class "radio" like the following:</p>
  
  <pre><code>            &lt;span class="input-nile"&gt;
                &lt;input type="radio" value="" name="user[nile_administration]" class="input__field input__field-nile radio" id="user_nile_administration_classinput__field_input__field-nile_radio"&gt;
                &lt;label class="radio-label"&gt;Nile Administration&lt;/label&gt;
              &lt;/span&gt;
  </code></pre>
  
  <p>How can i do that?</p>