Order properties consistently

Every time I start my GraphQL server, the order of all properties, enums and parameters as shown in [GraphiQL](https://github.com/graphql/graphiql) or as parsed with [GraphQL CLI](https://github.com/graphql-cli/graphql-cli) changes. These tools just take the order as it comes from the server and display it. I think `handler` should order all these properties alphabetically to have a consistent output of the callable schema at the endpoint where the `graphql-go` handler operates.