Set Accept header when fetching remote source

[MightyPixel]

Hi :)

What?
Explicitly set default Accept header to application/graphql-response+json or application/json.

Why?

1. The current code breaks if the subgraph responds with text (containing JSON)
   • This was the case when I tried adding a basic Quarkus subgraph (it uses smallrye-graphql internally).
   • It returns the following error:
     • Error: Couldn't load service definitions for "subgraph-poc" at http://localhost:8080/graphql: Expected JSON response body, but received: {"data":{"_service":{"sdl":"schema {\n query: Query\n...
2. This change also seems consistent with the draft spec of graphql

A valid workaround seems to be:

    supergraphSdl: new IntrospectAndCompose({
      subgraphs: [
      ...
      ],
      introspectionHeaders: {
        Accept: 'application/json',
      }
   })

Considerations

• Sorry if I've I'm missing anything!
• I didn't write tests since most of the RemoteGraphQLDataSource methods are private.

[apollo-cla]

@MightyPixel: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

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