Two interfaces on a node object produces incorrect graphql schema output

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* **What is the current behavior?**
Using two interfaces on a single node results in a schema file where the Interfaces are separated by comma and not ampersand. The comma results in an error, when using, say relay compiler.

* **If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem** via
a github repo, https://repl.it or similar.
- Inherit from two different interfaces
- Output the schema using graphql_schema

* **What is the expected behavior?**

* **What is the motivation / use case for changing the behavior?**

* **Please tell us about your environment:**

- Version:
- Platform:

* **Other information** (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow)

Reproduce the issue by inheriting from two interfaces and generating the schema with graphql_schema. Inspect the generated interface list and verify that the output uses ampersand-separated interfaces rather than commas, so tools such as the Relay compiler accept the schema.

Rédigé par le modèle d'indexation à partir du texte de l'issue.