How do I access the License API thru here?

Hello, according to Github API docs, you need to attach `application/vnd.github.drax-preview+json` in the `Accept` header. Once that is supplied, one should get the license info along with the repo object. However, I tried the following:

```
const gh = new Github({
username: 'xxxx',
password: 'xxxxxx'
}, '', 'application/vnd.github.drax-preview+json').search()
```

but to no avail. How can I access the License API using this package? Thanks in advance

Start by reviewing the Github constructor and search() call shown in the issue, then compare the package's request and header handling with the GitHub License API documentation. The work is done when this package can access license information through the documented API flow, with the supported usage made clear.

索引モデルが issue の本文から書いたものです。