local: Fix handling of single in eot_track (fixes #496)

- Adds test cases for code paths that caused bug
- Short circuits EOT next track handling when in single mode.
core: Reduce duplication between next and eot track handling.
Merge branch 'develop' into fix/bug-496-handle-single

Conflicts:
	mopidy/core/tracklist.py
core: Fix typos pointed out in PR#542
Merge pull request #542 from adamcik/fix/bug-496-handle-single

Resolve mistakes in single handling.