code: @Override
      public void addSessionData(final String key, final String value) {
          sessionData.put(key, value);
      }
docs: Add session data
  @param key the data key
  @param value the data value
  
  Note: while the FedoraSession interface permits multi-valued
  session data, this implementation constrains that to be single-valued.
  That is, calling obj.addSessionData("key", "value1") followed by
  obj.addSessionData("key", "value2") will result in only "value2" being associated
  with the given key.