1.5.1 can refuse to download new fragments just because "complete"

Created by: JibberJim

Another seeking issue with 1.5.1 is due to the PlaybackTimeRule refusing to propose a time because there's a "complete" fragment there, So if you play to the end, seeking into un-cached area can fail. Not sure why it didn't always fail, but possibly due to other seeking bugs where pending ones can be left behind and they could be rescheduled (see #881 (closed))