fixed bug in duplicate callback prevention code. h/t @Robert-Nelson
fixed bug in duplicate callback prevention code. h/t @Robert-Nelson

ludovicderochethales/node-github,Phanatic/node-github,meetearnest/node-github,kaizensoze/node-github,blamattina/node-github,trektech/test1,Gerhut/node-github,Dica-Developer/github-js,tempbottle/node-github,AkashSC/MyRepoDOMO,jamestalmage/node-github,eistrati/node-github,reviewninja/node-github,apiaryio/node-github,kaizensoze/github4,kstarsinic/node-github,onesfreedom/node-github,dfarr/node-github,ekristen/node-github,codeschool/node-github,cbargren/github-xhr