FlaskLoginClient does not work for @fresh_login_required views

[PoshAlpaca]

Describe the bug
When attempting to access a view that is protected by the @fresh_login_required decorator via FlaskLoginClient a 401 error is raised, even though the session's _fresh property is set to True by FlaskLoginClient.

To Reproduce
Steps to reproduce the behavior:

1. Create a Flask view function decorated with @fresh_login_required
2. Create a FlaskLoginClient and try to access /myview:

app.test_client_class = FlaskLoginClient
with app.test_client(user=my_user_obj, fresh_login=True) as client:
    response = client.get("/myview")
    assert response.status_code == 200 # fails because 401 != 200

3. 401 Unauthorized is returned even though session["_fresh"] is set to True

Expected behavior
The view function should be accessible, i.e. return a 200 OK because we have specified that the current session should be fresh.

Speculative explanation
FlaskLoginClient doesn't set _id on the session so when the fresh_login_required function accesses current_user on line 309 in utils.py, LoginManager._load_user is called which runs LoginManager._session_protection_failed which cannot find the _id property on session thus setting sess["_fresh"] = False.

See also issue #569 which discusses the behavior of invalidating a session when _id is not found.