Support entity protobufs

I thought Entity protobufs were required to have a key, but:

https://github.com/GoogleCloudPlatform/google-cloud-datastore/blob/6073e01540264db58ee50c30a1e9f006677eb794/proto/datastore_v1.proto#L174

and:

https://github.com/GoogleCloudPlatform/google-cloud-datastore/blob/6073e01540264db58ee50c30a1e9f006677eb794/proto/datastore_v1.proto#L117