azure.devops.v6_0.graph.models.PagedGraphUsers.as_dict() fails if continuation_token is not None

azure-devops==6.0.0b2

If continuation_token response on clients_v6_0.get_graph_client list_users() response is not None (i.e. multiple pages) then the as_dict() method on the PagedGraphUsers object fails:

  File "/opt/hostedtoolcache/Python/3.7.5/x64/lib/python3.7/site-packages/msrest/serialization.py", line 269, in as_dict
    return serializer._serialize(self, key_transformer=key_transformer, keep_readonly=keep_readonly)
  File "/opt/hostedtoolcache/Python/3.7.5/x64/lib/python3.7/site-packages/msrest/serialization.py", line 481, in _serialize
    new_attr = self.serialize_data(orig_attr, attr_desc['type'], **kwargs)
  File "/opt/hostedtoolcache/Python/3.7.5/x64/lib/python3.7/site-packages/msrest/serialization.py", line 707, in serialize_data
    data, data_type[1:-1], **kwargs)
  File "/opt/hostedtoolcache/Python/3.7.5/x64/lib/python3.7/site-packages/msrest/serialization.py", line 778, in serialize_iter
    raise SerializationError("Refuse str type as a valid iter type.")
msrest.exceptions.SerializationError: Refuse str type as a valid iter type

首先，在响应包含 continuation_token 时，通过 clients_v6_0.get_graph_client().list_users() 重现该失败，然后检查 PagedGraphUsers.as_dict() 以及 msrest 序列化 traceback。完成标准是：as_dict() 能够处理非 None 的 continuation token 而不会引发 SerializationError，并且针对多页响应场景有回归测试覆盖。