Object directive on root Mutation type doesn't have any effect

### What happened?
Placing a `OBJECT` directive on the root `Mutation` type doesn't get run

### What did you expect?
The directive should get run for the root nodes

### Minimal graphql.schema and models to reproduce
```gql
type Mutation @cacheControl(maxAge: 0) {
dummy: Boolean
}

directive @cacheControl(
maxAge: Int
) on OBJECT
```

### versions
- `gqlgen version`?
v0.13.0
- `go version`?
1.13
- dep or go modules?