remove the overwritten header in AsyncHttpClient  constructor

when I call addHeader to change "Content-Type" header,the  issue emerges.

like this:

POST /xampp/ HTTP/1.1
Content-Length: 99
Content-Type: application/x-www-form-urlencoded
Host: 192.168.22.199
Connection: Keep-Alive
Accept-Encoding: gzip
Content-Type: application/json