GRPCClient Ping method does not check health response

I've been working on a Node based plugin and was testing to see if my health service is working, and I noticed that the current GRPCClient implementation for `Ping()` doesn't actually evaluate if the service is healthy (`SERVING`):

https://github.com/hashicorp/go-plugin/blob/cfdf485783602a2ca85502dedebf441be7bcbc8d/grpc_client.go#L127-L134

Instead of just checking for `err` (which would tell you if the service was "hard down", or didn't implement `/grpc.health.v1.Health/Check`), this should also check the response from `client.Check` to check that it returned `grpc_health_v1.HealthCheckResponse_SERVING`.

Changing this would probably break a lot of people's stuff right now, but I thought I'd point it out.