private static String headerToString(final Headers headers) {
            if (headers == null) {
                return "";
            }
            return CollectionStream.of(headers.names()).map(n -> n + ":" + prepareHeaderValueForLog(n, headers.get(n))).toJoinedString(";");
        }
Add headers to HTTP request.

@param request   the request builder to add headers to
@param headers   the headers to add (in addition to the standard headers), can be null
@param cacheFile if non-null, the file to take ETag and If-Modified-Since information from
main/src/main/java/cgeo/geocaching/network/Network.java