flush every chunk only for chunked responses

let the servlet container buffer the response.
Based on feedback from @jroper:
https://github.com/yanns/play2-war-plugin/commit/b580b8c9000526aff2d1c6ab07443123fe109177#commitcomment-6312809
https://github.com/yanns/play2-war-plugin/commit/b580b8c9000526aff2d1c6ab07443123fe109177#commitcomment-6312832