Do not loose references when calling at_download script

The problem: references to file subpath make the reference engine to fail (reference lost or not created). The simplest example is an URL that call `http://plone/path/to/atfile/at_download/file`.

@hoka already fixed this issue in version 1.5.4, but his fix is not working for me because, even if resolveUID is used, the HTML catch by the reference engine is already transformed by outputfilters.
The problem is that `FSPythonScript` (like `at_download` is on Plone 4) implements the `IItem` interface.