ENUM types allow returning values other than the specified values

### What happened?

The server allows returning values for enums that aren't in the listed values for the enum types.

### What did you expect?

Spec doesn't allow this https://spec.graphql.org/June2018/#sec-Enums -> Result Coercion:

"GraphQL services must return one of the defined set of possible values. If a reasonable coercion is not possible they must raise a field error."

### Minimal graphql.schema and models to reproduce

Add an enum to the default schema

```graphql
type Todo {
...
type: TodoType!
}

enum TodoType {
WORK
PLAY
}
```

With resolver

```go
func (r *todoResolver) Type(ctx context.Context, obj *model.Todo) (model.TodoType, error) {
return model.TodoType("rubbish"), nil
}
```

### versions

- `gqlgen version` : 0.13.0
- `go version` : go version go1.13.15 darwin/amd64
- dep or go modules : modules