Return type name in resolveType

[patrick91]

Closes #1310

In this PR we updated GraphQL.js to 16 which removed the support for returning a GraphQLObjectType from resolveType.

I've changed the code to return the type name directly :)

The error was this:

{
  "errors": [
    {
      "message": "Support for returning GraphQLObjectType from resolveType was removed in [email protected] please return type name instead.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "hero"
      ]
    }
  ],
  "data": {
    "hero": null
  }
}

and is visible here: https://graphql.org/learn/queries/

[Urigo]

thank you @patrick91 !