FilePlayer getSecondsLoaded doesn't account for multiple buffered ranges

[mryarbles]

getSecondsLoaded () {
if (this.player.buffered.length === 0) return 0
return this.player.buffered.end(0)
}

As soon as a user seeks that buffered object will have a second range. The index 0 range will be fixed and the second range will now receive the updates, which leads to an incorrect or misleading value in progress events.

[cookpete]

Fixed by #380