Entity.save() should not muck with connection.transaction().add_auto_id()

[This code](https://github.com/GoogleCloudPlatform/gcloud-python/blob/41f97e187173e9f36930258bfd9e54048d22fe97/gcloud/datastore/entity.py#L229) should be inside 'connection.save_entity()' instead.
