How to get working with mergeSchemas
Hello,
I am trying to get this working with graphql-tool's mergeSchemas feature. Here is a contrived example:

```
import { GraphQLServer } from 'graphql-yoga'
import {
  mergeSchemas,
  makeExecutableSchema,
} from 'graphql-tools';

const UploadScalar = makeExecutableSchema({
  typeDefs: `
    scalar Upload
    type Query {
      _uploadTest: Boolean
    }
    type Mutation {
      _uploadTest: Boolean
      updateProfilePicture(
        picture: Upload!
      ): Boolean
    }
  `,
  resolvers: {
    Mutation: {
      updateProfilePicture(_, args) {
        console.log(args)
        return {
          success: true
        }
      }
    }
  }
})
  const server = new GraphQLServer({
    schema: mergeSchemas({
      schemas: [UploadScalar]
    })
  })
  server.start({
    port: 5444
  }, () => console.log('Server is running on http://localhost:5444'))
```
Basically, the Upload does not get forwarded to the schema and resolvers. I just get an empty picture object from the args.

If I use UploadScalar directly for the schema (const server = new GraphQLServer({ schema: UploadScalar })), it works. Of course, I don't do this in a real-world scenario. I would be merging multiple remote schemas which may use apollo-upload-server.

Does anyone know of a way I could work around this?