Fix wrong Http object used for JGitHub service provider

It seems that the wrong Http object is used in the service provider:

```
Argument 2 passed to Joomla\Github\Github::__construct() must be an instance of 
Joomla\Github\Http, instance of Joomla\Http\Http given
```

Just to be sure that it's not a local Linux thingy or something... here is a PR :wink: