title: Multiple Select Sending Empty String to Rails Controller
body: <p>I have a <code>tradie</code> model that has many <code>tradie_category</code> models. In the form for the tradie I added the following:</p>
  
  <pre><code>= form_for @tradie do |tradie|
    = tradie.select :tradie_category_ids, @categories, {}, { multiple: true }
  </code></pre>
  
  <p>In the HTML, the following is generated...</p>
  
  <pre><code>&lt;form accept-charset="UTF-8" action="/add_a_tradies/create" class="new_tradie" method="post"&gt;
    &lt;input name="tradie[tradie_category_ids][]" type="hidden" value=""&gt;
    &lt;select class="form-control" id="tradie_tradie_category_ids" multiple="multiple" name="tradie[tradie_category_ids][]"&gt;
      &lt;option value="2"&gt;mechanic&lt;/option&gt;
      &lt;option value="1"&gt;plumber&lt;/option&gt;
    &lt;/select&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>I removed the authenticity token and submit button because it's not necessary with my problem. Whenever I do post the form, the params <code>tradie_category_ids</code> is returned as an empty string. What's wrong and how do I fix it so I can created multiple tradie categories for the tradie that is created.?</p>