Artist is missing from Song.to_dict

[johnwmillr]

https://github.com/johnwmillr/LyricsGenius/blob/a0595147727a9e1932b0fec0f8529b9956bdbcf1/lyricsgenius/song.py#L162-L166

artist should be exported along with the other Song fields.

[allerter]

In #162, all we would need to do is this:

def to_dict(self):
    body = super().to_dict()
    body['artist'] = self.artist
    body['lyrics'] = self.lyrics
    return body

[johnwmillr]

Perfect!