Fix issue where seeders improperly marked as leechers

[sidd]

This fixes an issue I was running into with traditional BitTorrent clients. The tracker was marking seeders as leechers, even though this was not the case.

When starting a new transfer with all of the files already downloaded, a started event is sent out with left=0 (at least this is the case in Deluge and Transmission). A completed event is explicitly not sent out (FTA https://wiki.theory.org/BitTorrentSpecification):

completed: Must be sent to the tracker when the download completes. However, must not be sent if the download was already 100% complete when the client started. Presumably, this is to allow the tracker to increment the "completed downloads" metric based solely on this event.

Currently params.left is set to Infinity even if it was supplied as left=0 in the querystring. This fixes that issue, and retains the behavior of setting params.left to Infinity when it is unparseable.

[feross]

Looks good to me. I'm just going to swap isNaN for Number.isNaN since I usually try to avoid isNaN's automatic coercion. Doesn't make a difference in this case, though.

[feross]

8.0.13