Invalid VirtualService in networking/v1alpha3/virtual_service.proto

When I apply the VirtualService "ratings-route" from https://preliminary.istio.io/docs/reference/config/networking/v1alpha3/virtual-service/ and _networking/v1alpha3/virtual_service.proto_ there is a validation error:

```
bad Duration: time: unknown unit d in duration 1d
```

Caused by 
```
//       allowHeaders:
//       - X-Foo-Bar
//       maxAge: "1d"
```