private static void addHeaders(final Builder request, @Nullable final Parameters headers, @Nullable final File cacheFile) {
        for (final ImmutablePair<String, String> header : Parameters.extend(Parameters.merge(headers, cacheHeaders(cacheFile)))) {
            request.header(header.left, header.right);
        }
    }
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