added an explaining comment

Serializer/GenericSerializationVisitor.php

@@ -84,6 +84,8 @@ abstract class GenericSerializationVisitor extends AbstractVisitor
             $this->root = array();
             $rs = &$this->root;
         } else {
+            // ArrayObject is specially treated by the json_encode function and
+            // serialized to { } while a mere array would be serialized to [].
             $rs = isset($type['params'][1]) ? new \ArrayObject() : array();
         }