Fix issues opening multi-files at the same time.

Property stream changed to getStream to work with MultiStream's function mode to use one file at time.
Also add path.normalize to fix tests comparing files path in Windows (non / path separator).

Obs.: it's nothing I'm used to do, but I have left the old stream code commented.
Actually, that would create a closure over `input` which might hold onto the data longer than needed. This is fine.