title: How to select default option in rails 4 select_tag with multiple true
body: <p>I want to select default string when no option is selected. I tried the following but its not working</p>
  
  <pre><code>&lt;%=select_tag "users", options_from_collection_for_select(User.all, "id", "name",""),prompt: "select user",:multiple =&gt; true%&gt;
  </code></pre>
  
  <p>I am not getting any selected value, how to achieve it.</p>