protect Object.keys use against null
protect Object.keys use against null

desmondmorris/node-twitter,timisbusy/ntwitter,cesaregb/ntwitter,gabfusi/node-twitter,FiftyThree/node-twitter,SteveUpton/ftwitter,rneilson/node-twitter,AvianFlu/ntwitter,santiperez/ntwitter,Shugardude/ntwitter2