title: Bootstrap mr-auto doesn't work
body: <p>i am using bootstrap4 , and this is my code, I add  class <code>mr-auto</code> in the checkbox line, so i think the <code>New use?Sign up now!</code> should be at the right ,but it doesn't , what's the problem?</p>
  
  <pre><code>&lt;h1 class="text-center"&gt;Log in&lt;/h1&gt;
  &lt;div class="row"&gt;
  &lt;div class="col-md-6 offset-md-3"&gt;
    &lt;form action="/login" accept-charset="UTF-8" method="post"&gt;&lt;input name="utf8" type="hidden" value="&amp;#x2713;" /&gt;&lt;input type="hidden" name="authenticity_token" value="bhbKzszsGeADlAPkfoW4U58mGd9zfOy9mIl62R/gODqAFXevFsmTj//StPsLnrf8XpNlpkR+BHY6pgxlSlh8Og==" /&gt;
      &lt;div class="form-group"&gt;
        &lt;input type="text" name="email" id="email" class="form-control" placeholder="用户名/Email" autofocus="autofocus" /&gt;
      &lt;/div&gt;
      &lt;div class="form-group"&gt;
        &lt;input type="password" name="password" id="password" class="form-control" placeholder="password" /&gt;
      &lt;/div&gt;
      &lt;input type="submit" name="commit" value="login" class="btn btn-primary btn-block" data-disable-with="login" /&gt;
  
      &lt;div class="form-check mt-2 mr-auto d-inline-block"&gt;
        &lt;label class="form-check-label p-0"&gt;
          &lt;input type="checkbox" name="remember_me" id="remember_me" value="{:class=&amp;gt;&amp;quot;form-check-input &amp;quot;}" /&gt; Remember me
        &lt;/label&gt;
      &lt;/div&gt;
      &lt;p class=" d-inline-block"&gt;New user?&lt;a href="/signup"&gt;Sign up now!&lt;/a&gt;&lt;/p&gt;
      &lt;/form&gt;    
    &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>