Fix displaying of duration when loading transcoded media

When adding a transcoded music file (e.g. `*.flac`, `*.wav`) to the queue and start playing it, the duration on the bottom-right corner (part of jplayer) is always `00:00`. This only happen to transcoded music formats and doesn't happen to browser-supported formats like mp3 and ogg. I suppose this is because `transcodedURL()` doesn't copy the original `track`'s `duration` field to the new `track` object.