Fix a bug with auto-prefix on Windows.

On Unix, dirname('/x') returns /. But on Windows, it returns \. This
causes a very stupid bug which did not appear with /x/y because on Windows
returns, it returns /x. Yes.

Thanks @thehawk and @mathias-strasser!
[auto] Merge from Router

    • #45df265 — Fix a bug with auto-prefix on Windows. (Ivan Enderlin, 2013-06-21)

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Merge remote branch 'library-router/master'