Usage:   request_header_replace header_name message
	Example: request_header_replace User-Agent Nutscrape/1.0 (CP/M; 8-bit)

	This option allows you to change the contents of headers
	denied with request_header_access above, by replacing them
	with some fixed string.

	This only applies to request headers, not reply headers.

	By default, headers are removed if denied.