构造函数的参数和默认选项合并

function extend(){
               var obj={},
                arg=argments,
                len=arg.length;
                
                for(var i=0;i<len;i++){

                    for(var key in arg[i]){

                        obj[key]=arg[i][key];
                 }  
               };
                   return obj;
                                    }