title: Opensaml error receiving correct unmarshaller
body: <p>I am running this code at opensaml2.6</p>
  
  <pre><code>Element metadataRoot = document.getDocumentElement();
  
  // Unmarshall
  UnmarshallerFactory unmarshallerFactory = Configuration.getUnmarshallerFactory();
  Unmarshaller unmarshaller = unmarshallerFactory.getUnmarshaller(metadataRoot);
  if (unmarshaller == null) {
      System.out.println("Error receiving unmarshaller for this document.");
      return;
  }
  </code></pre>
  
  <p>For the document</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;&lt;saml2:EncryptedAssertion xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion"&gt;
  </code></pre>
  
  <p><code>Unmarshaller</code> return a <code>null</code>, Can you help me to undertand how are the criterias to lookup the correct <code>Unmarshaller</code> and how this works at opensaml?</p>