code: public void addHeaderValue(String headerName, String headerValue) {
  		Objects.requireNonNull(headerName, "headerName cannot be null");
  		Objects.requireNonNull(headerValue, "headerValue cannot be null");
  
  		getHeaderValues(headerName).add(headerValue);
  	}
docs: Adds the given header value.
  Note that {@code headerName} and {@code headerValue} cannot be null.
  @param headerName the name of the header
  @param headerValue the value to add for the header
  @throws NullPointerException if either parameter is {@code null}