[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.

# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s schema"""