title: Select2 with Bootstrap theme not honoring input group class
body: <p>I'm trying to use the select2 with an input group, but the button is always on the next line after the select2 control.  If you remove the select2 class it works as expected.  Why does the select2 bootstrap not honor the standard bootstrap input group classes if it's based on the bootstrap theme?  What am I missing to ensure they are both on the same line and displayed as a group?</p>
  
  <p>Without select2 class (and how I would like it to look)
  <a href="https://i.stack.imgur.com/D05jc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/D05jc.png" alt="enter image description here"></a></p>
  
  <p>With Select2</p>
  
  <p><a href="https://i.stack.imgur.com/Zjnbh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Zjnbh.png" alt="enter image description here"></a></p>
  
  <p>With Select2 and adding style="width:75%" (off at bottom)
  <a href="https://i.stack.imgur.com/HFREN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HFREN.png" alt="enter image description here"></a></p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;head&gt;
      &lt;link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css"&gt;
      &lt;link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.2/css/select2.min.css" /&gt;
      &lt;link rel="stylesheet" href="https://select2.github.io/select2-bootstrap-theme/css/select2-bootstrap.css" /&gt;
      &lt;link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.6.3/css/all.css"&gt;
  
      &lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"&gt;&lt;/script&gt;
      &lt;script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.bundle.min.js"&gt;&lt;/script&gt;
      &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.2/js/select2.full.js"&gt;&lt;/script&gt;
  &lt;/head&gt;
  &lt;body class="container"&gt;
      &lt;p&gt;&lt;/p&gt;
      &lt;div class="input-group"&gt;
          &lt;select class="form-control select2" placeholder="Search"&gt;&lt;/select&gt;
          &lt;div class="input-group-append"&gt;
              &lt;button class="btn btn-success" type="submit"&gt;Go&lt;/button&gt;
          &lt;/div&gt;
      &lt;/div&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  
  &lt;script&gt;
      $(".select2").select2({
          theme: "bootstrap",
          placeholder: "Search"
  } );
  &lt;/script&gt;
  </code></pre>