metadata: multiple custom tags are not processed properly

[bll123]

When specifying multiple custom tags, only the last custom tag is applied. The namespace seems to be immaterial -- only the last custom tag is applied. This can be seen with the command line tool, but is also the case using the API.

bll-g7:bll$ LD_LIBRARY_PATH=../plocal/lib ../plocal/bin/mp4tag \
    --set $'\xa9alb:S:album' --set $'\xa9ART:S:artist' \
     --set BDJ4/aaa:S:aaa --set BDJ4/bbb:S:bbb \
     ../test-music/010-tango-m4a.m4a  out.m4a
bll-g7:bll$ LD_LIBRARY_PATH=../plocal/lib ../plocal/bin/mp4tag out.m4a 
Tool:[S] Lavf58.45.100
AlbumArtist:[S] 010-albartist
GenreName:[S] Rock
Name:[S] 010-tango-m4a
Album:[S] album
Artist:[S] artist
BDJ4/bbb:[S] bbb