So the default content type header was not recognized properly by [GraphQL gateway](https://github.com/apollographql/federation/blob/%40apollo/gateway%402.5.4/gateway-js/src/datasources/RemoteGraphQLDataSource.ts#L299). I tried to [submit PR](https://github.com/apollographql/federation/pull/2769) but they said it doesn't conform to the [GraphQL spec](https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md#serialization-format).