title: Apache Commons Configurations containsKey returns false on existing key
body: <p>Apologies if this is documented, but I wasn't able to find this on the official documentation.</p>
  
  <p>I'm using <code>XMLConfiguration</code> to load this file, but <code>config.containsKey ("aliases")</code> returns false and <code>config.containsKey ("aliases.alias")</code> returns true. Am I forced to check for "aliases.alias" or is there a way to determine if "aliases" exists as node?  </p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;configuration&gt;
      &lt;name&gt;Vocabulary&lt;/name&gt;
      &lt;aliases&gt;
          &lt;alias&gt;Dictionary&lt;/alias&gt;
      &lt;/aliases&gt;
      &lt;extensions&gt;
          &lt;ext&gt;dic&lt;/ext&gt;
          &lt;ext&gt;txt&lt;/ext&gt;
          &lt;ext&gt;pdf&lt;/ext&gt;
          &lt;ext&gt;doc&lt;/ext&gt;
      &lt;/extensions&gt;
      &lt;scanmode&gt;1&lt;/scanmode&gt;
  &lt;/configuration&gt;
  </code></pre>