Fix sponsor key field extraction
When extracting key field there is no sponsor or user object, instead
only their link names are known.
# Directions for web crawlers.
# See http://www.robotstxt.org/wc/norobots.html.
User-agent: HTTrack
User-agent: puf
User-agent: MSIECrawler
User-agent: Nutch
Disallow: /