This PR fixes a bug where the schema definition block schema { } is used. If there happened to be a type called Mutation or Subscription, but not included in the schema definition block, it would be mistakenly set as an operation.

This PR follows on from this proposed spec improvement graphql/graphql-spec#987 and corresponding fix in the JS reference implementation https://github.com/graphql/graphql-js/pull/3839/files.

Here's Benjie's virus schema example:

type Query {
  viruses: [Virus!]
}

type Virus {
  name: String!
  knownMutations: [Mutation!]!
}

type Mutation {
  name: String!
  geneSequence: String!
}

schema {
  query: Query
}

The schema definition block/keyword defines query as the only operation in this schema. We have a type Mutation but that's referring to a virus, not the GraphQL operation. (Health warning: the word schema is terribly overloaded.)

Previous behaviour was

• Operation type definitions only recognise query as an operation - this was correct
• Generated GraphQLSchema sets the mutationType - this was not correct, and is fixed by this PR