Keep operations w/ scalar-type returns

[nVitius]

Right now, any mutations or queries defined with a scalar return type will be omitted from the generated operations. This patch allows these to be generated.

e.g.

type mutation {
  deleteEntity(entityId: ID!): Boolean
}

should compile to:

mutation deleteEntity($entityId: ID!) {
  deleteEntity(entityId: $entityId)
}