set json charset US-ASCII to UTF-8

Characters will be garbled if the JSON request containing a multi-byte character. 
It is for a hipchat server to misunderstand the character code of a request.

'Content-Type: application/json' -> charset default(US-ASCII)
'Content-TYpe: application/json; charset=utf-8' -> charset UTF-8

For example, if a multi-byte character is called by hipchatter, a hipchat server will receive an unjust demand.

`json:true` and please set a default to charset=utf-8.