Make MP3 extractor include nmm:albumArtist info
Make MP3 extractor include nmm:albumArtist info

This property wasn't being set in nmm:MusicAlbum classes.
Fixes GB#641906 - albumArtist information is not provided.

outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker