[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.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl