Added possibility to rename fields on a DataSet.get()

Added functionality to extend the fields array, to a fields object,
similar to the type declaration. This makes it possible rename
properies when you get them.

lib/DataSet.js

@@ -664,8 +664,14 @@ DataSet.prototype._filterFields = function (item, fields) {
   var filteredItem = {};
 
   for (var field in item) {
-    if (item.hasOwnProperty(field) && (fields.indexOf(field) != -1)) {
-      filteredItem[field] = item[field];
+  	if(Array.isArray(fields)){
+  		if (item.hasOwnProperty(field) && (fields.indexOf(field) != -1)) {
+  			filteredItem[field] = item[field];
+   	   }
+    }else{
+      	if (item.hasOwnProperty(field) && fields.hasOwnProperty(field)) {
+  			filteredItem[fields[field]] = item[field];
+   	   }  
     }
   }