Misc XMP

[bep]

• xmp: Add support for additional metadata
• xmp: Adjust type handling etc. for XMP child elements

[LNA-DEV]

I did take a look at it. Seems pretty good to me 👍

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My observations

There are two issues with the tests:

1. The mushroom.jpg.json file needs to be regenerated because you changed the file.
2. The firstUpper function does not work correctly. The following did work for me.

func firstUpper(s string) string {
	if s == "" {
		return s
	}
	runes := []rune(s)
	runes[0] = unicode.ToUpper(runes[0])
	return string(runes)
}