[Decoding] MusicData not decoding properly

[jsbean]

The music-data element in MusicXML is a collection of any number of values within a set of possible types

<!ENTITY % music-data
	"(note | backup | forward | direction | attributes |
	  harmony | figured-bass | print | sound | barline | 
	  grouping | link | bookmark)*">

This is representable as an enum with associated values in Swift (really, it's a union type):

enum MusicDatum {
    case note(Note)
    case attributes(Attributes)
    ...
}

Due to CoreOffice/XMLCoder#25, arrays of enums with associated values are not yet decodable.

[jsbean]

This is closed by MaxDesiatov/XMLCoder #119.