[File] Fix relative paths calculation for mixed slash paths

So, hopefully the last pull request for windows-tests issues.
When paths were added from the directory with the recursive directory iterators there were some mixed paths constructed like: `vfs://project/src/Foo\Bar.php`. They were not converted, so the diff-tests (behat) fail.
I fixed this edge case. 

second: I noticed that the psr-0-classname test did not work on windows with unix paths.

and third: the master tests to fail right now, thats why this pull requests fails, too.
Sorry, could not figure it out