Fix create issue validation errors when no labels are given

I'm seeing an (unexpected?) validation error from the API. I regenerated the cassette to demonstrate the 422 response in one commit, and then added a fix in the next commit. This is the error message returned from GitHub when the `labels` param is omitted:

```
Expected data to be of type "array"; value was: nil. // See: https://developer.github.com/v3/issues/#create-an-issue
```

I was under the impression that the `labels` param is supposed to be optional, but the API docs are a bit vague on that point.