'var_expand on invalid formats shouldn\'t raise'

def test_bad_var_expand(self):

    self.assertEqual(ip.var_expand(u"{'a':5}"), u"{'a':5}")
    self.assertEqual(ip.var_expand(u'{asdf}'), u'{asdf}')
    self.assertEqual(ip.var_expand(u'{1/0}'), u'{1/0}')
