Two interfaces on a node object produces incorrect graphql schema output

**Note: for support questions, please use stackoverflow**. This repository's issues are reserved for feature requests and bug reports.

* **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)