private static void populateHeaders(HeaderFields outputHeaders,
            ListMap<String, String> searchHeaders) {
        if (searchHeaders == null) {
            return;
        }
        for (Map.Entry<String, List<String>> header : searchHeaders.entrySet()) {
            for (String value : header.getValue()) {
                outputHeaders.add(header.getKey(), value);
            }
        }
    }
Copy custom HTTP headers from the search result over to the HTTP response.

@param outputHeaders the headers which will be sent to a client
@param searchHeaders the headers from the search result, or null
container-search/src/main/java/com/yahoo/search/handler/HttpSearchResponse.java