Sorry guys, but I think you are confusing things a little. [`Content-type`](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.17) and [`Accept`](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1) are two different headers. Why do you want to mix 'em up? Maybe L23 should look like:
```groovy
headers['Accept'] = accept()
```

where:
```groovy
String accept() {
    'application/vnd.github.v3+json'
}
```