'Check that openForWriting returns an object providing C{ftp.IWriteFile}.'

def test_openForWriting(self):

    d = self.shell.openForWriting(('foo',))
    def cb1(res):
        self.assertTrue(ftp.IWriteFile.providedBy(res))
        return res.receive().addCallback(cb2)
    def cb2(res):
        self.assertTrue(IConsumer.providedBy(res))
    d.addCallback(cb1)
    return d
