Download is stuck when choosing XAR as output and leaving the Target field empty on Jetty 8.1.9

Same as XWIKI-10201 but only on Jetty, works well for me on Tomcat.

As fas as I can see the stream is properly closed but the connection with the browser is still not released and looks like not everything has bee sent since the XAR on client side is not valide. For some reason closing the stream on jetty does not actually flush and close like on tomcat.