Check for required fields while parsing the schema

[k4n4ry]

fixes: #548

package main

import (
	"log"
	"net/http"

	graphql "github.com/graph-gophers/graphql-go"
	"github.com/graph-gophers/graphql-go/relay"
)

type query struct{}

func (_ *query) Hello(args struct{ ID *graphql.ID }) string { return "Hello, world!" }

func main() {
	s := `
                type Query {
                        hello(id: ID!): String!
                }
        `
	schema := graphql.MustParseSchema(s, &query{})
	http.Handle("/query", &relay.Handler{Schema: schema})
	log.Fatal(http.ListenAndServe(":8080", nil))
}

When running the above, the following error happen, and exit.

panic: field "ID" must be a non-pointer since the parameter is required

[pavelnikolov]

Thank you for your contribution!