<itemvalue="Performs the [action] on each header. The [action] function is called with each header's name and a list of the header's values. The casing of the name string is determined by the last [add] or [set] operation for that particular header, which defaults to lower-casing the header name unless explicitly set to preserve the case."/>
<itemvalue="Sets the header [name] to [value]. Removes all existing values for the header named [name] and then [add]s [value] to it."/>