code: public ChainingXmlWriter register(String parameterName, ICalDataType dataType) {
  		parameterDataTypes.put(parameterName, dataType);
  		return this;
  	}
docs: Registers the data type of a non-standard parameter. Non-standard
  parameters use the "unknown" data type by default.
  @param parameterName the parameter name (e.g. "x-foo")
  @param dataType the data type
  @return this