code: @Override
      public void setAttribute(String key, Object value) {
          if (value == null) {
              setAttribute(key, null);
          } else {
              setAttribute(key, Collections.singletonList(value));
          }
      }
docs: Sets the specified attribute to a value.
  
  <p>Reference implementation checks for the setting of the username attribute and updates the
  EntityIdentifier accordingly
  
  @param key Attribute's name
  @param value Attribute's value