[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.

from os.path import join
assert __file__.endswith(join('scripts', 'script1.py')), __file__
assert '__main__' == __name__, __name__
assert [] == __args__, __args__