gRPC call has problem with `Data` field

Hi,

I used gRPC for sending FCM push notification with `Data` field, but in my client side i received bad data structure! 

```
...,
"data": {
   "customfieldA": "{"Kind":{"StringValue":"1563021905599"}}",
   ...
}
```
What's `"{"Kind":{"StringValue":"1563021905599"}}"` ?! 

> i used README.md example for gRPC call.


Thanks.
