title: Serializing inside PgwModal returns empty values
body: <p>I want users to be able to register themselfs inside a modal (pgwmodal.js). Serializing the form returns keys with empty values even though they are filled in.</p>
  
  <p>HTML</p>
  
  <pre><code>&lt;div id="modal-register" style="display: none;"&gt;
      &lt;div class="row"&gt;
          &lt;form method="post"&gt;
              &lt;div class="input width_50"&gt;
                  &lt;label for="register_input_first_name"&gt;Name&lt;/label&gt;
                  &lt;input type="text" name="register_input_first_name" id="register_input_first_name"&gt;
                  &lt;input type="text" name="register_input_last_name" id="register_input_last_name"&gt;
              &lt;/div&gt;
              &lt;div class="input"&gt;
                  &lt;label for="register_input_email"&gt;Email&lt;/label&gt;
                  &lt;input type="text" name="register_input_email" id="register_input_email"&gt;
              &lt;/div&gt;
              &lt;div class="input"&gt;
                  &lt;label for="register_input_password"&gt;Password&lt;/label&gt;
                  &lt;input type="password" name="register_input_password" id="register_input_password"&gt;
              &lt;/div&gt;
              &lt;div class="input"&gt;
                  &lt;button id="button_register" type="button" class="button button-primary"&gt;Create user&lt;/button&gt;
              &lt;/div&gt;
          &lt;/form&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>jQuery</p>
  
  <pre><code>jQuery(document).ready(function($) {
  
      // create modal
      $('.register-trigger').click(function() {
          $.pgwModal({
              target: '#modal-register',
              title: 'New user',
              maxWidth: 970
          });
      });
  
      // serialize when button is clicked
      $(document).on("click", "#button_register", function() {
          console.log($('#modal-register form').serialize());
          $.post("functions/func_register.php",$('#modal-register form').serialize(),
          function(data) {
              console.log(data);
          }, 'json');
      });
  
  });
  </code></pre>
  
  <p>I think I somehow need to update #modal-register with the contents of the modal. If I fill in every field of the form, the serialize function give the following response:</p>
  
  <p>register_input_first_name=&amp;register_input_last_name=&amp;register_input_email=&amp;register_input_password=</p>