code: public void addAllHeaderValues(String headerName, List<String> headerValues) {
  		Objects.requireNonNull(headerName, "headerName cannot be null");
  		Objects.requireNonNull(headerValues, "headerValues cannot be null");
  
  		getHeaderValues(headerName).addAll(headerValues);
  	}
docs: Adds the list of header values for the given header.
  Note that {@code headerName} and {@code headerValues} cannot be null.
  @param headerName the name of the header
  @param headerValues the list of values to add for the header
  @throws NullPointerException if either parameter is {@code null}