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

Added test for issue
https://github.com/sass/node-sass/issues/522
Even better:

```javascript
[
  fixture('include-files/bar.scss').replace(/\\/g, '/'), 
  fixture('include-files/foo.scss').replace(/\\/g, '/'), 
  fixture('include-files/index.scss').replace(/\\/g, '/') 
].map(function(expect) {
  assert(stats.includedFiles.indexOf(expect) !== -1);
});

```