Always call `bar` when item is ready to play

Streaming AVPlayerItems will not have a `duration`, but we should still
start playing them when they indicate that they are ready to play. This
change leaves the duration setting code only to items with a valid
duration, but moves the call to `bar` so that it is always called when
an item is ready.
Merge pull request #11 from jballanc/master

Always call `bar` when item is ready to play