Allow string values to be None.
Allow string values to be None.

wong2/gunicorn,zhoucen/gunicorn,prezi/gunicorn,keakon/gunicorn,ccl0326/gunicorn,wong2/gunicorn,elelianghh/gunicorn,zhoucen/gunicorn,tempbottle/gunicorn,wong2/gunicorn,gtrdotmcs/gunicorn,WSDC-NITWarangal/gunicorn,alex/gunicorn,prezi/gunicorn,GitHublong/gunicorn,z-fork/gunicorn,alex/gunicorn,harrisonfeng/gunicorn,gtrdotmcs/gunicorn,mvaled/gunicorn,zhoucen/gunicorn,1stvamp/gunicorn,jamesblunt/gunicorn,malept/gunicorn,ccl0326/gunicorn,tejasmanohar/gunicorn,prezi/gunicorn,mvaled/gunicorn,alex/gunicorn,gtrdotmcs/gunicorn,pschanely/gunicorn,malept/gunicorn,ccl0326/gunicorn,ammaraskar/gunicorn,mvaled/gunicorn,jamesblunt/gunicorn,pschanely/gunicorn,malept/gunicorn,1stvamp/gunicorn,urbaniak/gunicorn,jamesblunt/gunicorn,MrKiven/gunicorn,beni55/gunicorn,pschanely/gunicorn,urbaniak/gunicorn,ephes/gunicorn,urbaniak/gunicorn,1stvamp/gunicorn