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`

问题出在 CollectFields() 函数中，可能位于代码生成或字段收集逻辑中。首先检查 gqlgen 中与字段收集相关的源代码，尤其是片段合并部分。查看测试套件中的类似案例，以了解预期行为。修复需要确保来自片段和内联选择的字段都能正确合并。

由索引模型根据 Issue 内容生成。