Sort RFC 8785 object properties by their keys

[ydah]

RFC 8785 §3.2.3 requires sorting property names by their UTF-16 code units, independently of locale.

The single block argument to Hash#sort_by receives [key, value], so the current code sorts by the pair's inspected representation. This produces incorrect ordering that also depends on Encoding.default_external.

JSON.generate({ "a" => 1, "a b" => 2 }, rfc8785: true)
# Before: '{"a b":2,"a":1}'
# After:  '{"a":1,"a b":2}'

Destructure the pair to sort by the key alone.