fix(GraphQL): This PR allow to use __typename in mutation. (#7285)

[JatinDev543]

Fixes GRAPHQL-921

The first mutation below will now return the "Mutation" type, which previously was giving an error.
Mutation:

mutation {
                       __typename
			addpost1(input: [{title: "Dgraph", numLikes: 92233720 }]) {
				post1 {
					title
					numLikes
				}
			}
		}

`Response:`
      {
                "__typename":"Mutation",
		"addpost1": {
			"post1": [{
				"title": "Dgraph",
				"numLikes": 92233720

			}]
		}
	}

(cherry picked from commit b6edc7e)

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This change is