Structured configs with Dict annotations allowing forbidden objects.

[pereman2]

Describe the bug
DictConfig with primitive type as element type is allowing other structured configs as a valid element.

To Reproduce

@dataclass
class A:
   pass
@dataclass
class C:
    foo: Dict[str, int] = field(default_factory=lambda: {"a": A()})

cfg = OmegaConf.structured(C) # Doesn't raise an error

Expected behaviour

It should raise an error when assigning structured configs to a different element type in DictConfigs.

context

• OmegaConf version: 2.0.2
• Python version: 3.6, 3.7 and 3.8
• Operating system : mac
• Please provide a minimal repro