Mutation response types are ignored since 3.8.5

[severin]

(This is maybe related to #272)

We use Federation and we have some mutations that contain an ErrorExtensions type in their results. This ErrorExtensions type is federated between multiple services.

When upgrading from 3.8.1 to 3.10.0 the ErrorExtensions type disappears from the _Entity union:

-union _Entity = ErrorExtensions | Other | Federated | Types
+union _Entity = Other | Federated | Types

The type is only "reachable" through mutations, so it's not really an orphan type.

Applying the hacky solution from #272 (adding an orphan_type declaration before query) seems to work around the issue though:

class OnRunningSchema < GraphQL::Schema
  include ApolloFederation::Schema
  federation version: '2.0'

  orphan_types [Types::ErrorExtensionsType]  

  query Types::QueryType

  # ...
end