Sort RFC 8785 keys after as_json conversion

[ydah]

Apply as_json before sorting keys by UTF-16 code units, and reuse the converted keys for output.

coder = JSON::Coder.new(rfc8785: true) { |object| object.year.to_s }
coder.dump({ Time.utc(1999) => 1, "1999!" => 2 })
# Before: {"1999!":2,"1999":1}
# After:  {"1999":1,"1999!":2}

Call the callback once per converted key and reject collisions when building the converted hash to avoid overwriting entries.