Fix return type annotation on `get_creation_date()`

[jwodder]

_DAVResource.get_creation_date() is annotated as returning Optional[datetime], but _DAVResources.get_property_value() passes the method's return value to get_rfc3339_time(), which requires a Unix timestamp instead.

[mar10]

Thanks for contributing. There is still a lint error that we need to fix before merging

[jwodder]

@mar10 Lint resolved.