Dict subclass data not preserved during assignment

[Jasha10]

Describe the bug
When a Dict subclass is assigned to an omegaconf Node, the data from the dict is thrown away.

To Reproduce

$ cat tmp.py
from dataclasses import dataclass
from typing import Dict
from omegaconf import OmegaConf

@dataclass
class Str2Str(Dict[str, str]):
    x: int = 0

a = Str2Str()
assert a.x == 0
a["y"] = 1
assert a["y"] == 1
cfg_a = OmegaConf.create(a)
assert cfg_a.x == 0
assert "y" in cfg_a
assert cfg_a["y"] == 1

$ python tmp.py
Traceback (most recent call last):
  File "/Users/lappy486/omegaconf/tmp.py", line 18, in <module>
    assert "y" in cfg_a
AssertionError

Expected behavior
We expect the above assertions to succeed.

Additional context

• OmegaConf version: 2.1.0dev20
• Python version: 3.9.1
• Operating system : MacOS

[Jasha10]

Filed based on comments here: #581 (comment)

[omry]

Yup, definitely smells like a bug.