Integration Tests Not Properly Mocked

I followed the integration tests example pattern in my own app.

```
// mock the datasources' underlying fetch methods, whether that's a REST
// lookup in the RESTDataSource or the store query in the Sequelize datasource
launchAPI.get = jest.fn(() => [mockLaunchResponse]);
```

However I noticed it was still making api calls to the rest endpoint. I then cloned this repo turned off my wifi and ran the integration tests and sure enough I got an error.

```
errors": Array [
+ Object {
+ "extensions": Object {
+ "code": "INTERNAL_SERVER_ERROR",
+ "exception": Object {
+ "code": "ENOTFOUND",
+ "errno": "ENOTFOUND",
+ "message": "request to https://api.spacexdata.com/v2/launches?flight_number=30 failed, reason: getaddrinfo ENOTFOUND api.spacexdata.com api.spacexdata.com:443",
+ "type": "system",
+ },
},
- "rocket": Object {
- "type": "FT",
- },
+ "locations": Array [
+ Object {
+ "column": 3,
+ "line": 2,
+ },
+ ],
+ "message": "request to https://api.spacexdata.com/v2/launches?flight_number=30 failed, reason: getaddrinfo ENOTFOUND api.spacexdata.com api.spacexdata.com:443",
+ "path": Array [
+ "launch",
+ ],
```

This tells me that `launchAPI.get` is not being properly mocked. Which I think would make sense as there is no `launchAPI.get` however there is a `launchAPI.prototype.get` however that doesn't seem to work when I change the tests to that.

Any ideas?

首先在禁用网络的情况下运行集成测试，并检查为 launchAPI.get 赋值的示例，然后追踪负责处理发射查询的 RESTDataSource 方法。issue 未指定文件或测试命令；完成的标准是集成测试能够离线通过，不请求 api.spacexdata.com，并且仍然返回预期的火箭数据。

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