Mutations are eventually converted to protobufs, we should use proto serialization here. Otherwise, this class has to be tested extensively.    I'm looking at   https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-spanner/src/main/java/com/google/cloud/spanner/Mutation.java#L337 and toProto method has package visibility. We need either expose to/from proto methods in the SDK, make Mutation serializable or replicate that code here.