Change ArraySerializer's initialization of DefaultSerializer to pass
:root => false in order to avoid adding root keys to each of the
items in the array. This ensures consistent results between serializing
arrays of objects with serializers and arrays of objects without
serializers. Fixes#495.
This is to ensure that PORO's as_json is called if no serializer
is specified.
Original behaviour was that serializable_hash was being called,
overriding the as_json method.
