code: public InputStream getAsStream(final Archive<?> archive) {
          final Archive<?> testable = findTestableArchive(archive);
          final Collection<Node> values = collectPersistenceXml(testable);
          if (values.size() == 1) {
              return values.iterator().next().getAsset().openStream();
          }
  
          return null;
      }
docs: Returns open stream of persistence.xml found in the archive, but
  only if single file have been found.
  
  @return Input stream of persistence.xml found or null if zero or multiple found in the archive.