gzipResponseWriter should also be a Flusher
gzipResponseWriter should also be a Flusher

To be consistent with 3faad41b437c48cea37863123fab425169bc0c6e and c64cf218b0a82d144e779454817eb0371a5002a8
joshix/caddy,tw4452852/caddy,pyed/caddy,tobya/caddy,pyed/caddy,pyed/caddy,dusty/caddy,elcore/caddy,elcore/caddy,ollie314/caddy,DenBeke/caddy,joshix/caddy,yukimochi/caddy,pedronasser/caddy,captncraig/caddy,tw4452852/caddy,pedronasser/caddy,tobya/caddy,DenBeke/caddy,DenBeke/caddy,dusty/caddy,cez81/caddy,pedronasser/caddy,joshix/caddy,dusty/caddy,captncraig/caddy,evermax/caddy,elcore/caddy,yukimochi/caddy,evermax/caddy,pyed/caddy,ollie314/caddy,dusty/caddy,ollie314/caddy,yukimochi/caddy,tw4452852/caddy,devangels/caddy,DenBeke/caddy,cez81/caddy,cez81/caddy,evermax/caddy,tw4452852/caddy,evermax/caddy,ollie314/caddy,pedronasser/caddy,devangels/caddy,tobya/caddy,tobya/caddy,devangels/caddy,captncraig/caddy