Added bulk update functionality and test

Bulk update functionality.

Allow updating all items in a list or queryset, and deleting any items that were not found.

    queryset = Book.objects.all()
    serializer = BookSerializer(queryset, data=request.DATA, many=True)
    if serializer.is_valid():
        serializer.save()  # Update instances in `request.DATA`, and delete any not referenced.