Fix a tiny but disastrous bug to make depth works as it should be
Fix a tiny but disastrous bug to make depth works as it should be

In old code, without this "null --> referrer" parameter, the depth - 1 passed in is treated as referrer, while depth parameter in _crawlUrl method becomes 'undefined', and eventually it makes the depth related control totally malfunctions.  I correct this bug by simply adding null. LMAO!
antivanov/js-crawler,antivanov/js-crawler,antivanov/js-crawler,antivanov/js-crawler