The getSong API endpoint is used by, e.g., the subsonic plugin to mopidy.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

If the name of an artist is not set, the serialiser will crash. Instead,
just skip such an artist when serialising a list of artists.

Also add test for serialising an artist with an empty name.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

A failed cast to int will raise ValueError, which is not currently caught
by the error checking code, leading to a crash. Fix this so a proper error
message can be returned.

Also add test for getting artist with non-numeric ID.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

This adds a metadata test case for extracting data from an opus test
file. The added file is just converted from test.ogg and the same
metadata fields are used.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>