Response is formatted in JSON disregard of download format

[dai-chen]

In our code, the response appears to be formatted by JSON.stringfy no matter what's the download format. This will corrupt the content in csv and txt because they're not JSON.

Code: https://github.com/opendistro-for-elasticsearch/sql-kibana-plugin/blob/415ace37b69112318087cec4eb0e2de46c437666/server/services/QueryService.ts#L34