Entity.reload does not work

[dhermes]

We should make sure to fix the tests as well as the code:

        key = self._must_key
        dataset = self._must_dataset
        entity = dataset.get_entity(key.to_protobuf())

(the key should be re-loaded, not the key protobuf).

Example failure:

>>> entity.reload()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "gcloud/datastore/entity.py", line 215, in reload
    entity = dataset.get_entity(key.to_protobuf())
  File "gcloud/datastore/dataset.py", line 139, in get_entity
    key = Key.from_path(*key_or_path)
TypeError: from_path() argument after * must be a sequence, not Key