Some issuers (e.g. PaperOS) emit `scope` as a JSON array (`[]` or
`["openid","profile"]`) instead of the RFC 6749 space-delimited string.
SpaceDelimited.UnmarshalJSON now accepts both forms; a JSON array
is converted to the equivalent slice. Other non-string, non-array
values still return an error.
Adds test cases: array_values and array_empty.
