[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.
include *.pyinclude */*.pyrecursive-include data *.gif *.png *.ico *.css *.jsrecursive-include i18n *.porecursive-include wdoc *