Added test for an issue found in the output of stats.includedFiles

Added test for issue
https://github.com/sass/node-sass/issues/522
Yup! That makes sense. See https://github.com/sass/libsass/issues/634.

Also, note that `.render` or `.renderSync` doesn't make difference for this test. For example:

```javascript
var stats = {};
require("./lib/").renderSync({
   data: fs.readFileSync('c:/temp/alpha/index.scss','utf8'),
   stats: stats,
   includePaths: ['c:/temp/alpha']
});

stats
```

This^ would yield the same result.