Fix artwork and poster methods for playlists

[JonnyWong16]

Description

Playlist.key is in the format /playlist/<ratingKey> instead of /library/metadata/<ratingKey>. The former does not work with resources like artwork and posters.

Ref: 4094678

Fixes #991

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable