BUG: TimedeltaIndex slicing may reset freq
`DatetimeIndex` preserve freq after slicing.

```
di = pd.date_range('2001', '2005', freq='D')
di[1:5:2]
# DatetimeIndex(['2001-01-02', '2001-01-04'], dtype='datetime64[ns]', freq='2D', tz=None)
```

But `TimedeltaIndex` doesn't.

```
tdi = pd.timedelta_range('1day', '5day', freq='D')
tdi[1:5:2]
# TimedeltaIndex(['2 days', '4 days'], dtype='timedelta64[ns]', freq=None)
```
