DatetimeIndex.normalize doesn't clear ns field
```
import pandas as pd
import numpy as np
didx = pd.DatetimeIndex(np.array([1380585623454345752, 1380585612343234312]).astype("datetime64[ns]"))
didx.normalize()
```

the output is:

```
<class 'pandas.tseries.index.DatetimeIndex'>
[2013-10-01 00:00:00.000000752, 2013-10-01 00:00:00.000000312]
Length: 2, Freq: None, Timezone: None
```
