enhanced test to spot a file upload bug

https://github.com/globaleaks/GlobaLeaks/issues/638

and it's something not very easy to be tested, as described in the comment:

    class TestAdminStaticFile(helpers.TestHandler):
    """
    Sadly we can't use the official handler test, because in a
    file upload, Cyclone and GL patches transform the body in a StringIO.

    So the code in GLBackend expect a StringIO. If here I use the handlers,
    with a StringIO inside, can't be JSON-ized.

    If I send a real dict to the handler, the GLBackend code fail, because
    expect a StringIO.

    If I support in the handler both kind of data, well, no. I would not
    change the handler code just to fit our test.

    So... that's shit, but _post hanlder is tested in a more direct way
    """
    _handler = admstaticfiles.StaticFileCollection