title: Rails form_for multiple collection_select, values not selected on form failure
body: <p>I'm not sure why, but my form is not showing the options selected on submit, even though the params hash shows that the information is being returned to the page. </p>
  
  <p>Collection select code:</p>
  
  <pre><code>&lt;%= f.collection_select :post_topic_ids, PostTopic.all, :id, :name, {}, { multiple: true, class: 'form-control' } %&gt;
  </code></pre>
  
  <p>Which renders:</p>
  
  <pre><code>&lt;select multiple="multiple" class="form-control" name="post[post_topic_ids][]" id="post_post_topic_ids"&gt;
    &lt;option value="1"&gt;Psychology&lt;/option&gt;
    &lt;option value="2"&gt;Engineering&lt;/option&gt;
    &lt;option value="3"&gt;Nanotechnology&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>Params returned after form validation error</p>
  
  <pre><code>params = {"post"=&gt;{"post_topic_ids"=&gt;["", "1"]}}
  </code></pre>
  
  <p><strong>Update</strong> </p>
  
  <p>I have also tried:</p>
  
  <pre><code>&lt;%= select_tag 'post_topic_ids', options_for_select(PostTopic.all.collect{ |p| [p.name, p.id] }), multiple: true %&gt;
  </code></pre>
  
  <p>and:</p>
  
  <pre><code>&lt;%= select_tag 'post_topic_ids', options_from_collection_for_select(PostTopic.all, "id", "name"), multiple: true %&gt;
  </code></pre>
  
  <p>Which renders:</p>
  
  <pre><code>&lt;select name="post_topic_ids[]" id="post_topic_ids" multiple="multiple"&gt;&lt;option value="1"&gt;Psychology&lt;/option&gt;
  &lt;option value="2"&gt;Engineering&lt;/option&gt;
  &lt;option value="3"&gt;Nanotechnology&lt;/option&gt;&lt;/select&gt;
  </code></pre>