Can't create a blob

```JavaScript
const GitHub = require('github-api');
const github = new GitHub({
username: 'iddan',
password: '********',
});
const mesimaivrit = github.getRepo('mesimaivrit');
mesimaivrit.createBlob('hello')
.then(console.log)
```
throws: `Error: 404 error making request POST https://api.github.com/repos/mesimaivrit/git/blobs: "Not Found"`

Start at the createBlob entry point in the JavaScript wrapper and reproduce the supplied example against the GitHub git/blobs endpoint. Trace the request made by createBlob and compare it with the endpoint's expected request, then verify that the example creates a blob successfully without the reported 404.

索引モデルが issue の本文から書いたものです。