shared/json: reject non-utf-8 strings

shared/json: reject non-utf-8 strings

JSON strings must be utf-8-clean. We also verify this in json_parse_string()
so we would reject a message with invalid utf-8 anyway.

It would probably be slightly cheaper to detect non-conformaning strings in
serialization, but then we'd have to fail serialization. By doing this early,
we give the caller a chance to handle the error nicely.

The test is adjusted to contain a valid utf-8 string after decoding of the
utf-32 encoding in json ("विवेकख्यातिरविप्लवा हानोपायः।", something about the
cessation of ignorance).