BUG: tm.assert_index_equal ignores TimedeltaIndex.freq
```
tdi = pd.TimedeltaIndex(['1 Day', '2 Days'], freq='D')
tdi2 = tdi._shallow_copy(freq=None)

tm.assert_index_equal(tdi, tdi2)
```

The call to assert_index_equal should raise AssertionError, doesn't.