cannot unmarshal object google.protobuf.Timestamp

```
% grpcurl -version
grpcurl 1.9.3
```
```
syntax = "proto3";

package demo;

option go_package = "./demo;demo";

import "google/protobuf/timestamp.proto";

service Demo {
rpc Test (TestRequest) returns (TestResponse);
}

message TestRequest {
google.protobuf.Timestamp date_of_birth = 1;
string name = 2;
}

message TestResponse {
string result = 1;
}
```
simple go-grpc server
```
package main

import (
"context"
"fmt"
"log"
"net"

"test/demo"

"google.golang.org/grpc"
"google.golang.org/grpc/reflection"
)

type server struct {
demo.UnimplementedDemoServer
}

func (s *server) Test(ctx context.Context, req *demo.TestRequest) (*demo.TestResponse, error) {
dob := "nil"
if req.DateOfBirth != nil {
dob = req.DateOfBirth.AsTime().String()
}
return &demo.TestResponse{
Result: fmt.Sprintf("Hello, %s! Date of birth: %s", req.Name, dob),
}, nil
}

func main() {
lis, err := net.Listen("tcp", ":8082")
if err != nil {
log.Fatalf("listen: %v", err)
}
grpcServer := grpc.NewServer()
demo.RegisterDemoServer(grpcServer, &server{})
reflection.Register(grpcServer)
fmt.Println("gRPC listening on :8082")
if err := grpcServer.Serve(lis); err != nil {
log.Fatalf("serve: %v", err)
}
}

```

```
% protoc --version
libprotoc 32.1

% grpcurl -plaintext -d '{"name":"Ivan", "date_of_birth": {"seconds": 820454400, "nanos": 0}}' localhost:8082 demo.Demo/Test
Error invoking method "demo.Demo/Test": error getting request data: json: cannot unmarshal object into Go value of type string

```

Start by reproducing the failure with the supplied demo proto, Go server, and grpcurl command against demo.Demo/Test. Compare the documented google.protobuf.Timestamp input with the failing object form; done means the request is accepted and the RPC receives the timestamp without the JSON unmarshalling error.

Written by the indexing model from the issue text.