title: Select multiple using Rails `#select` form helper
body: <p>I've got a fairly simple (it seems) bit of code to create a multiple-selection element:</p>
  
  <pre><code>&lt;%= form_for @post do |f| %&gt;  
    #stuff
  
    &lt;%= fields_for :tags |tag_fields| %&gt;
      &lt;%= tag_fields.label :select_tags %&gt;
      &lt;%= tag_fields.select :tags, Tag.all, multiple: true %&gt;
    &lt;% end %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>For some reason when it renders the page, the <code>multiple: true</code> part isn't getting parsed, and the form is just rendering as a generic dropdown with only one option selectable. What am I missing?</p>
  
  <p>(Rails version is 5.0.0 in case relevant)</p>