downloader: Fix bug where mark_working wasn't called

This meant that old workers would never shut down, causing us to fetch the same media playlist
and same segments multiple times for no reason, and to never give up in face of (non-403/404) errors
even once we have something else working.

downloader/downloader/main.py

@@ -321,6 +321,7 @@ class StreamWorker(object):
 
 			# We successfully got the playlist at least once
 			first = False
+			self.manager.mark_working(self)
 
 			# Start any new segment getters
 			date = None # tracks date in case some segment doesn't include it