datastore: Arrayize input field names.

Author: rakyll

lib/datastore/query.js

@@ -61,8 +61,9 @@ Query.prototype.order = function(order) {
 };
 
 Query.prototype.groupBy = function(fieldNames) {
+  var fields = util.arrayize(fieldNames);
   var q = util.extend(this, new Query());
-  q.groupByVal = fieldNames;
+  q.groupByVal = fields;
   return q;
 };