__delitem__ not as expected for enum key-type DictConfig

[Jasha10]

Given a config with Enum key-type:

from enum import Enum
from omegaconf import DictConfig

class Enum1(Enum):
    FOO = 1
    BAR = 2

cfg = DictConfig({Enum1.FOO: 'enum key'}, key_type=Enum1)

normally it is possible to use either the enum value Enum1.FOO or the str value "FOO" for access:

>>> cfg[Enum1.FOO]
'enum key'
>>> cfg["FOO"]
'enum key'

But using string access fails when __delitem__ is called:

>>> del cfg["FOO"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jasha10/omegaconf/omegaconf/dictconfig.py", line 405, in __delitem__
    del self.__dict__["_content"][key]
KeyError: 'FOO'

The __delitem__ works as expected when called with an enum value:

>>> del cfg[Enum1.FOO]
>>> assert Enum1.FOO not in cfg

Expected behavior:

The __delitem__ method should behave the same as __setitem__ and __getitem__: when a DictConfig instance has enum-typed keys, either a string or an enum member should be usable to specify what index will be deleted.