Get to 100% branch coverage

Exercise edge cases in branch statements.
Comments in the protobuf spec[1] say:

    // At most one of name or ID may be set.

I would still argue for the change, as the original implies a priority (id over name) which is not mandated in the spec, and we end up correctly mapping the protobuf's contents if the bits on the wire don't match the comment.

[1]  https://github.com/GoogleCloudPlatform/google-cloud-datastore/blob/6073e01540264db58ee50c30a1e9f006677eb794/proto/datastore_v1.proto#L58