When resource is finished playing, position should be at end of resource

After a resource has finished playing, the position() reported should be at the end of the resource, not 0. This is common behaviour (e.g. in HTML5, in common players etc). Playing again should start from the start, of course.

It might be possible to fix this high up in the stack - if status is EndOfMedia, return the last position/duration etc.

Unit tests may need to be updated.