fix(GraphQL): Fix custom(dql: ...) with __typename (GRAPHQL-1098)

[abhimanyusinghgaur]

This PR fixes the working of @custom(dql: ...) with __typename and fragments.

Bug Description:

1. Apply GraphQL schema:

interface Node {
 id: ID!
}
type User implements Node {
 name: String
}
type Token implements Node {
 key: String
}
type Query {
 queryNodeR: [Node] @custom(dql: """
  query {
   queryNodeR(func: type(Node)) {
     dgraph.type
     id: uid
     name: User.name
     key: Token.key
   }
  } """)
}

2. Add data:

mutation {
 addUser(input: [{name: "User1"}]) {numUids}
 addToken(input: [{key: "Token1"}]) {numUids}
}

3. Query with both the auto-generated and the custom query and see the difference:

query {
 queryNode {
 id
 __typename
 ... UserFrag
 ... TokenFrag
 }
 queryNodeR {
 id
 __typename
 ... UserFrag
 ... TokenFrag
 }
}
fragment UserFrag on User {
 name
}
fragment TokenFrag on Token {
  key
}

4. The response being returned was:

{
  "data": {
    "queryNode": [
      {
        "id": "0x2",
        "__typename": "User",
        "name": "User1"
      },
      {
        "id": "0x3",
        "__typename": "Token",
        "key": "Token1"
      }
    ],
    "queryNodeR": [
      {
        "id": "0x2",
        "__typename": "Node",
        "name": "User1",
        "key": null
      },
      {
        "id": "0x3",
        "__typename": "Node",
        "name": null,
        "key": "Token1"
      }
    ]
  }
}

queryNodeR should have had the same result as queryNode.

---

This change is 

[vmrajas]

LGTM