code: private String addAttributeValue(String currentValue, String valueToAdd)
      {
          String newValue;
          if (currentValue == null || currentValue.length() == 0) {
              newValue = "";
          } else {
              newValue = currentValue + " ";
          }
          return newValue + valueToAdd;
      }
docs: Add an attribute value to an existing attribute. This is useful for example for adding a value to an HTML CLASS
  attribute.
  
  @param currentValue the current value of the attribute (can be null)
  @param valueToAdd the value to add
  @return the current value augmented by the value to add