Using type with name `Subscription` results in funky behaviour.

Describe the bug When defining a ObjectType with the name Subscription, and either having no Subscriptions definitions or having a Subscription root with a different Subscription root type results in errors.

To Reproduce Steps to reproduce the behavior:

1. Use the following schema file:

schema {
  query: QueryRoot
}

type Subscription {
  name: String
  rank: Float
}

type QueryRoot {
  commander: Commander
}

1. Use the following document:

query getWelcomeGreet {
  welcome {
    hello
  }
}

1. Run gql-gen with the following codegen.yml:

schema:
  - 'src/app/**/*.graphqls'
documents:
  - 'src/**/*.graphql'
config: {}
generates:
  src/app/graphql/queries.ts:
    config: {}
    plugins:
      - 'typescript-common'
      - 'typescript-client'
      - 'typescript-apollo-angular'
watch: true
overwrite: true
require: []

Expected behavior Expected a normal generation of types, queries etc.

[kamilkisiela]

v0.14.5 is out 😃

[ardatan]

Great!