title: Setting option value as array index in a select with rails
body: <p>I want to create a select tag using rails <code>collection_select</code> or <code>options_for_select</code> with the items of an array which are not model based. The value of the option should be the array index of each item. </p>
  
  <p>Example: desired output for <code>['first', 'second', 'third']</code>:</p>
  
  <pre><code>&lt;select id="obj_test" name="obj[test]"&gt;
      &lt;option value="0"&gt;first&lt;/option&gt; 
      &lt;option value="1"&gt;second&lt;/option&gt; 
      &lt;option value="2"&gt;third&lt;/option&gt; 
  &lt;/select&gt; 
  </code></pre>