code: protected void preserve(Map<String, String> output, String name, String value) {
  		if (prefix != null) {
  			output.put(prefix + name, value);
  		}
  	}
docs: add a header {@code prefix + name} with value {@code value} to {@code output}.
  if {@code prefix} is either null or empty, this method is no-op.
  @param output headers Map
  @param name header name
  @param value header value