make sure a listener is inheritable
make sure a listener is inheritable

fix #978

elelianghh/gunicorn,keakon/gunicorn,harrisonfeng/gunicorn,GitHublong/gunicorn,MrKiven/gunicorn,malept/gunicorn,prezi/gunicorn,ccl0326/gunicorn,ephes/gunicorn,prezi/gunicorn,gtrdotmcs/gunicorn,malept/gunicorn,mvaled/gunicorn,mvaled/gunicorn,zhoucen/gunicorn,gtrdotmcs/gunicorn,gtrdotmcs/gunicorn,tempbottle/gunicorn,mvaled/gunicorn,malept/gunicorn,prezi/gunicorn,ccl0326/gunicorn,zhoucen/gunicorn,WSDC-NITWarangal/gunicorn,zhoucen/gunicorn,z-fork/gunicorn,ccl0326/gunicorn,tejasmanohar/gunicorn