Drop trailing spaces from files in normalise on Windows

[ndmitchell]

writeFile "foo    " ""
writeFile "bar" ""
doesFileExist "foo" -- True
doesFileExist "foo   " -- True
doesFileExist "bar   " -- True

Seemingly any trailing spaces in a path are ignored by Windows. Note that directories behave the same way if they are the last argument, but if you add a path separator, then they don't get their spaces removed. Very weird.