permit empty header values, similar to @mipaaa's fork
permit empty header values, similar to @mipaaa's fork

The change to handling %O following %b's rules is NOT included, as I've added proper support for %b.
kassner/log-parser,skynet/log-parser,mipaaa/apache-log-parser,Sophist-UK/Sophist_log-parser,andrelotto/log-parser,gregarious-repo/log-parser