title: align-middle not working in table
body: <p>I have problem, that align-middle class not working with bootstrap 4. How can I fix this problem? Here is my code:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="false" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://code.jquery.com/jquery-3.2.1.slim.min.js"&gt;&lt;/script&gt;
  &lt;link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" rel="stylesheet"/&gt;
                              &lt;div class="table table-responsive"&gt;
                                  &lt;table id="search_content" class="table table-bordered"&gt;
                                      &lt;tbody&gt;
                                          &lt;tr class="d-flex"&gt;
                                              &lt;td class="col-md-4 align-middle"&gt;Pirkimo numeris&lt;/td&gt;
                                              &lt;td class="col-md-8 align-middle" colspan="4"&gt;&lt;input id="numerisId" class="form-control" name="numeris" type="text"/&gt;&lt;/td&gt;
                                          &lt;/tr&gt;
                                          &lt;tr class="d-flex"&gt;
                                              &lt;td class="col-md-4 align-middle"&gt;Pavadinimas&lt;/td&gt;
                                              &lt;td class="col-md-8 align-middle" colspan="4"&gt;&lt;input id="pavadinimasId" class="form-control" name="pavadinimas" type="text" /&gt;&lt;/td&gt;
                                          &lt;/tr&gt;
                                          &lt;tr class="d-flex"&gt;
                                              &lt;td class="col-md-12 text-right"&gt;
                                                  &lt;input id="ieskoti" class="btn btn-danger" title="Ieškoti" type="Submit" value="Ieškoti" /&gt;
                                                  &lt;input id="trinti" class="btn btn-default" title="Išvalyti" type="submit" value="Išvalyti" /&gt;
                                              &lt;/td&gt;
                                          &lt;/tr&gt;
                                      &lt;/tbody&gt;
                                  &lt;/table&gt;
                              &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>Thanks for help in advance! :)</p>