fix: Metadata.serialize() sortAs and undefined language tag

[SifAa]

I discovered that when calling Manifest.serialize() it did not return correctly for sortAs in metadata and that undefined language did not use a correct BCP-47 tag. This PR should fix it.

[JayPanoz]

So funny story, I will have to check if there is any side-effect re. undefined → und cos’ of the following:

const test = { undefined: "unknown", en: "english", fr: "french" }
test["en"] // Returns "english"
test[undefined] // Note the lack of quotes. Returns "unknown"

So I’ll have to check pretty much all packages for this, to see whether things accidentally worked because undefined was used instead of und when we didn’t have a defined value.