Aliased pointer types become pointer addresses

[atombender]

Test case:

package main

import (
	"context"
	"encoding/json"
	"fmt"
	"log"

	"github.com/graphql-go/graphql"
)

type Value string

func main() {
	s, err := graphql.NewSchema(graphql.SchemaConfig{
		Query: graphql.NewObject(graphql.ObjectConfig{
			Name: "RootQuery",
			Fields: graphql.Fields{
				"value": &graphql.Field{
					Type: graphql.String,
					Resolve: func(p graphql.ResolveParams) (interface{}, error) {
						v := Value("hello")
						return &v, nil
					},
				},
			},
		}),
	})
	if err != nil {
		log.Fatal(err)
	}

	result := graphql.Do(graphql.Params{
		Schema:        s,
		RequestString: "query MyQuery { value } ",
		Context:       context.TODO(),
	})

	fmt.Printf("%#v\n", result.Data)
}

This prints

map[string]interface {}{"value":"0xc00000f500"}

instead of the expected:

map[string]interface {}{"value":"hello"}

Annoyingly, there's no way to work around this by implementing MarshalJSON on Value, since it seems graphql-go doesn't use it.

I'll see if I can create a PR.

Related to #199.