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)

通过继承两个接口并使用 graphql_schema 生成 schema 来复现该问题。检查生成的接口列表，并验证输出使用以连字符号分隔的接口，而不是使用逗号分隔，以便 Relay compiler 等工具能够接受该 schema。

由索引模型根据 Issue 内容生成。