CollectFields() not merging Fragment with custom addition

### What happened?
```graphql
fragment House on House {
id,state{foobar}
}

query getHouses {
houses{...House,state{barfoo}}
}
```

When using `CollectFields(ctx)` you get only the fields collected from `fragment House`.

### What did you expect?
A merge of given fragment and additional fields.

### Minimal graphql.schema and models to reproduce
```graphql
type Query {
houses: [House!]
}

type House {
id: ID
state: HouseState
}

type HouseState {
foobar: String
barfoo: String
}
```

### versions
- `gqlgen version`? `v0.12.2-dev`
- `go version`? `go1.15 windows/amd64`
- dep or go modules? `go modules`