settings_stub context manager will not remove settings that did not exist before

[gandhis1]

If there is a setting that does not exist, settings_stub will gladly add it, but will not remove it.

with settings_stub(SOME_SETTING_THAT_DOES_NOT_EXIST=1):
    print(settings.SOME_SETTING_THAT_DOES_NOT_EXIST)  # Prints 1
print(settings.SOME_SETTING_THAT_DOES_NOT_EXIST)  # Prints 1 instead of raising AttributeError

It ought to remove the setting if it did not exist originally.