fix a bug:  Memory-fs.join has a different behavior comparing to node's path.join when the non-first argument contains / or \. And it'll cause a bug when filename contains / or \(only webpack is all right).
fix a bug:
 Memory-fs.join has a different behavior comparing to node's path.join when the non-first argument contains / or \. And it'll cause a bug when filename contains / or \(only webpack is all right).

 so, at least, we should use node's path.join when w'll interact with real fs.

shama/webpack-stream