changed namespace iteration style

lib/rss.js

@@ -110,9 +110,9 @@ function generateXML (data){
         version: '2.0'
     };
 
-    for(var name in data.custom_namespaces) {
+    Object.keys(data.custom_namespaces).forEach(function(name) {
       _attr['xmlns:' + name] = data.custom_namespaces[name];
-    }
+    });
 
     //only add namespace if GeoRSS is true
     if(data.geoRSS){