WebAPI handles Chinese Characters / Emoji

I'm writing a WebAPI which required to handle HTTP POST and the body will include Chinese characters and emoji.  I wrote a loop back test but I found that the WebAPI returns "?" when I use cURL to test.  Is there any example or best practice on how to handle those kind of characters? 

a!localVariables(
  local!json: if(
    isnull(http!request.body),
    a!fromJson("{""hello"":""world 中文 😀""}"),
    a!fromJson(http!request.body)
  ),
  a!httpResponse(
    headers: {
      a!httpHeader(name: "Content-Type", value: "application/json")
    },
    body: a!toJson(local!json)
  )
)

% curl --location \
--request POST 'https://my.server.com/suite/webapi/LoopBack' \
--header 'Content-Type: application/json;charset=utf-8' \
--header 'Authorization: Bearer abc.def.ghi' \
--data-raw '{"hello":"world 中文 😀"}'

{"hello":"world ?? ?"}%