[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.
sync_schema_props_perms('State')
sync_schema_props_perms('state_of')
sync_schema_props_perms('BaseTransition')
sync_schema_props_perms('Transition')
sync_schema_props_perms('WorkflowTransition')
sync_schema_props_perms('transition_of')