title: Rails: datetime_field does not display control buttons
body: <p>I have a form, where I use <code>date_field</code> helper and it works well, the control buttons are shown, but when I change it with a <code>datetime_field</code> the field is displayed but control buttons are no displayed.</p>
  
  <p>My <code>date_field</code> which works fine:</p>
  
  <pre><code>=f.date_field :publishing_date, placeholder: "yyyy-mm-dd", class: "form-control"
  </code></pre>
  
  <p>the <code>datetime_field</code> which does not shown controls:</p>
  
  <pre><code>=f.datetime_field :publishing_date, class: "form-control"
  </code></pre>