title: Element attribute value is null even if the attribute has specific value
body: <p>Below is the part of html of a modal pop up in our website</p>
  
  <pre><code>&lt;div id="DeliveryDateModal" class="modal fade in" role="dialog" data-backdrop="static" data-keyboard="false" style="display: block; padding-right: 17px;" aria-hidden="false"&gt;
  </code></pre>
  
  <p>If the popup is displayed the attribute <code>aria-hidden</code> value is <code>true</code> and otherwise it is <code>false</code>.</p>
  
  <p>When I am trying to read the attribute value using Selenium webdriver </p>
  
  <pre><code>String value driver.findElement(By.id("DeliveryDateModal")).getAttribute("aria-hidden");
  </code></pre>
  
  <p>the string '<code>value</code>' is always <code>null</code>. Why is that it is null even though the attribute has a specific value</p>