default json charset US-ASCII to UTF-8

Characters will be garbled if the JSON request containing a multi-byte character. 
It is for a 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](https://www.npmjs.org/package/hipchatter), a hipchat server will receive an unjust demand. 

In `json:true`, please set a default to `charset=utf-8`.