Infinite recursiveness

[zvictor]

Gql-generator goes into infinite recursiveness once we add just 2 lines of code to the end of example/sampleTypeDef.graphql!
CC @RvanderLaan

Steps to reproduce:

1. Copy the code below to example/sampleTypeDef.graphql:

type Query {
    user(id: Int!): User!
    members: [Member!] @deprecated(reason: "Test a deprecated query")
}

type Mutation {
    signup(
        email: String!
        username: String!
        password: String!
    ): UserToken!

    signin(
        email: String!
        password: String!
    ): String!

    setConfig(
        prefs: PrefsInput
    ): Config!

    sendMessage(
        message: String!
    ): String! @deprecated(reason: "Test a deprecated mutation")
}

input PrefsInput {
    language: String!
}

type Config {
    language: String!
    level(domain: String!): Int!
    lastSeen(domain: String!): Int!
    theme(domain: String!): Int!
}

type Subscription {
    UserUpdated(UserId: Int!): User
}

type UserToken {
    token: String!
    user: User!
}

type User {
    id: Int!
    username: String!
    email: String!
    createdAt: String!
    context: Context!
    details: UserDetails!
    address: String! @deprecated(reason: "Test a deprecated field")
}

union UserDetails = Guest | Member | Premium

type Guest {
    region(language: String): String!
}

type Member {
    address: String!
}

type Premium {
    location: String!
    card: Card!
}

type Card {
    number: String!
    type: [CardType!]!
    owner: User! # <------------------ ADDED ---------------------
}

type CardType {
    key: String!
}

type Context {
    user: User! # Circular ref
    domain: String!
    card: Card! # <------------------ ADDED ---------------------
}

2. Run the tests or just node index.js --schemaFilePath ./example/sampleTypeDef.graphql --destDirPath ./example/output

[zvictor]

I think I solved it! 🎉
@timqian please decide on #59 so that I can base the solution for this issue on that refactoring or not.
Once #59 is merged I will push the fix for this one 😊