Fix DecodedCFDatetimeArray was being incorrectly indexed.

This was causing an error in the following situation:
```
ds = xray.Dataset()
ds['time'] = ('time', [np.datetime64('2001-05-01') for i in range(5)])
ds['variable'] = ('time', np.arange(5.))
ds.to_netcdf('test.nc')
ds = xray.open_dataset('./test.nc')
ss = ds.indexed(time=slice(0, 2))
ss.dumps()
```
Thanks @shoyer for the fix.