BC breaking when for "allow_objects=True" flag

[xvjiarui]

Describe the bug
In 2.2.1 version, "allow_objects=True" flag for DictConfg behavior changed.

To Reproduce

from omegaconf import OmegaConf, DictConfig
from collections import namedtuple
Shape=namedtuple("Shape", field_names=["channels", "height", "width"])
a = Shape(10, 2, 3)
cfg = DictConfig(dict(shape=a), flags={"allow_objects": True})

In 2.1.1:
cfg is {'shape': Shape(channels=10, height=2, width=3)}.

In 2.2.1:
cfg is [10, 2, 3].

[Jasha10]

Thanks for the bug report, @xvjiarui.

[xvjiarui]

Thank you! @Jasha10

[pixelb]

git bisect identifies commit 4b19378 as the issue