[datafeed] Set headers parameter to an empty dict in DataFeedParser.retrieve_url This parameter is then passed to urllib2.Request, which expects a dict and won't check for None. Closes #4842333.

if versions_map['cubicweb'][0] == (3, 7, 4):
    config['http-session-time'] *= 60
    config['cleanup-session-time'] *= 60
    config['cleanup-anonymous-session-time'] *= 60