Use explicit `many=True` in PaginationSerializer on `object_serializer` instantiation

Since implicit serialization of many objects without `many=True` is no longer valid (AFAIR it was depraceted in 2.3.*) `PaginationSerializer`  should use this flag to serialize each of it's pages.

Without this fix page serialization will fall if these conditions are met:
* pagination serializer has custom `object_serializer_class` Meta opt
* custom `object_serializer_class` has any fields
* `list` (or any other iterable with __len__ atribute) was passed to Paginator.

In result it is then impossible to serialize objects that are not models with pagination.