IN operator with `__key__` property does not work

Hello,
Sorry I'm not 100% if this should be a bug report or a feature request, but since the IN operator is working on the Datastore UI on the GCP Console I'm guessing it's a bug.

#### Environment details
I'm not allowed to share those, but I don't think it matters in this case

- OS type and version:
- Python version: `python --version`
- pip version: `pip --version`
- `google-cloud-datastore` version: `pip show google-cloud-datastore`

#### Steps to reproduce

1. Try to execute a query on the `__key__` property with the IN operator
2. Error : `ValueError: Invalid key: "[, ]"`

#### Code example

```python
query = client.query(kind=kind)
query.add_filter("__key__", "IN", [client.key(kind, key), client.key(kind, key2)])
```

#### Stack trace
```
# example
```
Sorry I'm not allowed to share the whole stack trace, but I investigated a bit and it seems to come from this condition :
```
if property_name == KEY_PROPERTY_NAME and not isinstance(value, Key):
raise ValueError('Invalid key: "%s"' % value)
```
In case `value` is a `List`, it should check that all elements from the list are Keys instead of raising an error

Making sure to follow these steps will guarantee the quickest resolution possible.

Thanks!