Don't cache if caching not allowed in Cache-Control HTTP header

### Issue description
Playing live HLS stream causes audioTrackUnderrun error, playback stops, and throws PlaylistStuckException (source error).

### Reproduction steps
Launch video. Playback will proceed normally for ~10 seconds and then start to stutter and eventually stop.

### Link to test content
Emailed.

### Version of ExoPlayer being used
dev-v2 branch, commit #d4eb2e5b8571f57169126a155c98af775f14b4ad

### Device(s) and version(s) of Android being used
* Samsung S6 Edge Plus running 6.0.1
* Nexus 9 Tablet running 6.0
* HTC One running 5.0.1

### A full bug report captured from the device
Emailed.